Power semiconductor devices with trenched shielded split gate transistor and methods of manufacture

ABSTRACT

A semiconductor power device includes a drift region of a first conductivity type, a well region extending above the drift region and having a second conductivity type opposite the first conductivity type, active trenches extending through the well region and into the drift region where the active trenches define an active area. Inside each of the active trenches is formed a first conductive gate electrode disposed along and insulated from a first trench sidewall, a second conductive gate electrode disposed along and insulated from a second trench sidewall, and a conductive shield electrode disposed between the first and second conductive gate electrodes, wherein the shield electrode is insulated from and extends deeper inside the trench than the first and second conductive gate electrodes. The device also includes source regions having the first conductivity type formed inside the well region and adjacent the active trenches. Electrical contact to the conductive shield electrode can be made inside the active area. The device can also include a perimeter trench extending at least partially around the active trenches such that at least some of the active trenches are perpendicular to the perimeter trench, gate fingers extending from a perimeter gate poly runner located in said perimeter trench, and shield poly fingers extending from a perimeter shield poly runner located in the perimeter trench. The gate fingers are staggered with respect to the shield poly fingers.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of the following commonly-assignedU.S. patent application:

Ser. No. 11/026,276, entitled “Power Semiconductor Devices and Methodsof Manufacture,” by Challa et al., filed Dec. 29, 2004;

which claims the benefit of the following provisionally-filed U.S.patent application:

No. 60/533,790, entitled “Power Semiconductor Devices and Methods ofManufacture,” by Challa et al. filed Dec. 30, 2003;

and is a continuation-in-part of the following commonly-assigned U.S.patent applications:

Ser. No. 10/640,742, entitled “Improved MOS Gating Method for ReducedMiller Capacitance and Switching Losses,” by Kocon et al., filed Aug.14, 2003 (now U.S. Pat. No. 6,870,220);

Ser. No. 10/442,670, entitled “Structure and Method for Forming a TrenchMOSFET Having Self-Aligned Features,” by Herrick, filed May 20, 2003(now U.S. Pat. No. 6,916,745);

and is related to the following commonly-assigned U.S. patentapplications:

Ser. No. 10/155,554, entitled “Field Effect Transistor and Methods ofits Manufacture,” by Mo et al., filed May 24, 2002 (now U.S. Pat. No.6,710,406);

Ser. No. 10/209,110, entitled “Dual Trench Power MOSFET,” by Sapp, filedJul. 30, 2002 (now U.S. Pat. No. 6,710,403);

Ser. No. 09/981,583, entitled “Semiconductor Structure with ImprovedSmaller Forward Loss and Higher Blocking Capability,” by Kocon, filedOct. 17, 2001 (now U.S. Pat. No. 6,677,641);

Ser. No. 09/774,780, entitled “Field Effect Transistor Having a LateralDepletion Structure,” by Marchant, filed Jan. 30, 2001 (now U.S. Pat.No. 6,713,813);

Ser. No. 10/200,056, entitled “Vertical Charge Control SemiconductorDevice with Low Output Capacitance,” by Sapp et al., filed Jul. 18, 2002(now U.S. Pat. No. 6,803,626);

Ser. No. 10/288,982, entitled “Drift Region Higher Blocking LowerForward Voltage Drop Semiconductor Structure,” by Kocon et al., filedNov. 5, 2002 (now U.S. Pat. No. 7,132,712);

Ser. No. 10/315,719, entitled “Method of Isolating the Current Sense onPlanar or Trench Stripe Power Devices while Maintaining a ContinuousStripe Cell,” by Yedinak, filed Dec. 10, 2002 (now U.S. Pat. No.6,906,362);

No. 10/222,481, entitled “Method and Circuit for Reducing Losses inDC-DC Converters,” by Elbanhawy, filed Aug. 16, 2002 (now U.S. Pat. No.6,930,473);

Ser. No. 10/235,249, entitled “Unmolded Package for a SemiconductorDevice,” by Joshi, filed Sep. 4, 2002 (now U.S. Pat. No. 6,740,541); and

Ser. No. 10/607,633, entitled “Flip Chip in Leaded Molded Package andMethod of Manufacture Thereof,” by Joshi et al., filed Jun. 27, 2003(now U.S. Pat. No. 6,949,410).

All of the above-listed applications are hereby incorporated byreference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates in general to semiconductor devices and inparticular to various embodiments for improved power semiconductordevices such as transistors and diodes, and their methods ofmanufacture, including packages and circuitry incorporating the same.

The key component in power electronic applications is the solid stateswitch. From ignition control in automotive applications tobattery-operated consumer electronic devices, to power converters inindustrial applications, there is a need for a power switch thatoptimally meets the demands of the particular application. Solid stateswitches including, for example, the power metal-oxide-semiconductorfield effect transistor (power MOSFET), the insulated-gate bipolartransistor (IGBT) and various types of thyristors have continued toevolve to meet this demand. In the case of the power MOSFET, forexample, double-diffused structures (DMOS) with lateral channel (e.g.,U.S. Pat. No. 4,682,405 to Blanchard et al.), trenched gate structures(e.g., U.S. Pat. No. 6,429,481 to Mo et al.), and various techniques forcharge balancing in the transistor drift region (e.g., U.S. Pat. Nos.4,941,026 to Temple, 5,216,275 to Chen, and 6,081,009 to Neilson) havebeen developed, among many other technologies, to address the differingand often competing performance requirements.

Some of the defining performance characteristics for the power switchare its on-resistance, breakdown voltage and switching speed. Dependingon the requirements of a particular application, a different emphasis isplaced on each of these performance criteria. For example, for powerapplications greater than about 300-400 volts, the IGBT exhibits aninherently lower on-resistance as compared to the power MOSFET, but itsswitching speed is lower due to its slower turn off characteristics.Therefore, for applications greater than 400 volts with low switchingfrequencies requiring low on-resistance, the IGBT is the preferredswitch while the power MOSFET is often the device of choice forrelatively higher frequency applications. If the frequency requirementsof a given application dictate the type of switch that is used, thevoltage requirements determine the structural makeup of the particularswitch. For example, in the case of the power MOSFET, because of theproportional relationship between the drain-to-source on-resistanceR_(DSon) and the breakdown voltage, improving the voltage performance ofthe transistor while maintaining a low R_(DSon) poses a challenge.Various charge balancing structures in the transistor drift region havebeen developed to address this challenge with differing degrees ofsuccess.

Device performance parameters are also impacted by the fabricationprocess and the packaging of the die. Attempts have been made to addresssome of these challenges by developing a variety of improved processingand packaging techniques.

Whether it is in ultra-portable consumer electronic devices or routersand hubs in communication systems, the varieties of applications for thepower switch continue to grow with the expansion of the electronicindustry. The power switch therefore remains a semiconductor device withhigh development potential.

BRIEF SUMMARY OF THE INVENTION

The present invention provides various embodiments for power devices, aswell as their methods of manufacture, packaging, and circuitryincorporating the same for a wide variety of power electronicapplications. Broadly, one aspect of the invention combines a number ofcharge balancing techniques and other techniques for reducing parasiticcapacitance to arrive at various embodiments for power devices withimproved voltage performance, higher switching speed, and loweron-resistance. Another aspect of the invention provides improvedtermination structures for low, medium and high voltage devices.Improved methods of fabrication for power devices are provided accordingto other aspects of the invention. Improvements to specific processingsteps, such as formation of trenches, formation of dielectric layersinside trenches, formation of mesa structures, processes for reducingsubstrate thickness, among others are offered by various embodiments ofthe invention. According to another aspect of the invention, chargebalanced power devices incorporate temperature and current sensingelements such as diodes on the same die. Other aspects of the inventionimprove equivalent series resistance (ESR), or gate resistance, forpower devices, incorporate additional circuitry on the same chip as thepower device and provide improvements to the packaging of chargebalanced power devices.

These and other aspects of the invention are described below in greaterdetail in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional view of a portion of an exemplary n-typetrench power MOSFET;

FIG. 2A shows an exemplary embodiment of a dual trench power MOSFET;

FIG. 2B shows an exemplary embodiment for a planar gate MOSFET withsource shield trench structure;

FIG. 3A shows part of an exemplary embodiment of a shielded gate trenchpower MOSFET;

FIG. 3B illustrates an alternative embodiment for a shielded gate trenchpower MOSFET that combines the dual trench structure of FIG. 2A with theshielded gate structure of FIG. 3A;

FIG. 4A is a simplified partial diagram of an exemplary embodiment of adual gate trench power MOSFET;

FIG. 4B shows an exemplary power MOSFET that combines a planar dual gatestructure with trenched electrodes for vertical charge control;

FIG. 4C shows an exemplary implementation of a power MOSFET thatcombines the dual gate and shielded gate techniques inside the sametrench;

FIGS. 4D and 4E are cross-sectional diagrams of alternative embodimentsfor a power MOSFET with deep body structure;

FIGS. 4F and 4G illustrate the impact of trenched deep body structureson the distribution of potential lines inside the power MOSFET near thegate electrode;

FIGS. 5A, 5B and 5C are cross-sectional diagrams showing portions ofexemplary power MOSFETs with various vertical charge balancingstructures;

FIG. 6 shows a simplified cross-sectional view of a power MOSFET thatcombines an exemplary vertical charge control structure with a shieldedgate structure;

FIG. 7 shows a simplified cross-sectional view of another power MOSFETthat combines an exemplary vertical charge control structure with a dualgate structure;

FIG. 8 shows one example of a shielded gate power MOSFET with verticalcharge control structure and integrated Schottky diode;

FIGS. 9A, 9B and 9C depict various exemplary embodiments for powerMOSFETs with integrated Schottky diode;

FIGS. 9D, 9E and 9F illustrate exemplary layout variations forinterspersing Schottky diode cells within the active cell array of apower MOSFET;

FIG. 10 provides a simplified cross-sectional view of an exemplarytrench power MOSFET with buried diode charge balancing structure;

FIGS. 11 and 12 show exemplary embodiments for power MOSFETs thatcombine shielded gate and dual gate techniques with buried diode chargebalancing, respectively;

FIG. 13 is a simplified cross-sectional view of an exemplary planarpower MOSFET that combines the buried diode charge balancing techniquewith integrated Schottky diode;

FIG. 14 shows a simplified embodiment of an exemplary accumulation-modepower transistor with alternate conductivity regions arranged inparallel to the current flow;

FIG. 15 is a simplified diagram of another accumulation-mode device withtrenched electrodes for charge spreading purposes;

FIG. 16 is a simplified diagram of an exemplary dual trenchaccumulation-mode device;

FIGS. 17 and 18 show other simplified embodiments for exemplaryaccumulation-mode devices with dielectric-filled trenches havingopposite-polarity exterior liner;

FIG. 19 is another simplified embodiment for an accumulation-mode devicethat employs one or more buried diodes;

FIG. 20 is a simplified isometric view of an exemplary accumulation-modetransistor that includes heavily-doped opposite polarity region alongthe surface of the silicon;

FIG. 21 shows a simplified example of a super-junction power MOSFET withalternating opposite-polarity regions in the voltage sustaining layer;

FIG. 22 shows an exemplary embodiment for a super-junction power MOSFETwith opposite-polarity islands non-uniformly spaced in the verticaldirection in the voltage sustaining layer;

FIGS. 23 and 24 show exemplary embodiments for super-junction powerMOSFETs with dual gate and shielded gate structures, respectively;

FIG. 25A shows a top view of active and termination trench layout for atrench transistor;

FIGS. 25B-25F show simplified layout views of alternative embodimentsfor trench termination structures;

FIGS. 26A-26C are cross-sectional views of exemplary trench terminationstructures;

FIG. 27 shows an exemplary device with termination trenches having largeradius of curvature;

FIGS. 28A-28D are cross-sectional views of termination regions withsilicon pillar charge balance structures;

FIGS. 29A-29C are cross-sectional views of exemplary embodiments ofultra-high voltage devices employing super-junction techniques;

FIG. 30A shows an example of edge contacting for a trench device;

FIGS. 30B-30F show exemplary process steps in forming the edgecontacting structure for a trench device;

FIG. 31A is an example of an active area contact structure for multipleburied poly layers;

FIGS. 31B-31M show an exemplary process flow for forming an active areashield contact structure for a trench;

FIG. 31N is a cross-sectional view of an alternate embodiment for anactive area shield contact structure;

FIGS. 32A and 32B are layout views of an exemplary trench device withactive area shield contact structure;

FIGS. 32C-32D are simplified layout diagrams of two embodiments formaking contact to the perimeter trench in a trench device with brokentrench structure;

FIG. 33A is an alternate embodiment for contacting trenched shield polylayers in the active area;

FIGS. 33B-33M show an example of a process flow for contacting an activearea shield structure of the type shown in FIG. 33A;

FIG. 34 shows an epi layer having a spacer or buffer (barrier) layer toreduce thickness of epi drift region;

FIG. 35 shows an alternative embodiment for a device with a barrierlayer;

FIG. 36 shows a barrier layer employed at a deep body-epi junction tominimize epi layer thickness;

FIG. 37 is a simplified example of the well-drift region junction of atransistor employing a diffusion barrier layer;

FIGS. 38A-38D show a simplified process flow for an example of aself-aligned epi-well trench device with buried electrode;

FIGS. 39A-39B show an exemplary process flow for an angled well implant;

FIGS. 40A-40E show an example of a self-aligned epi well process;

FIGS. 40R-40U show a method for reducing substrate thickness;

FIG. 41 shows an example of a process flow using a chemical process asthe final thinning step;

FIGS. 42A-42F show examples of improved etch processes;

FIGS. 43A and 43B show embodiments of a trench etch process thateliminates the bird's beak problem;

FIGS. 44A and 44B show alternative etch processes;

FIG. 45A-45C show a process of forming an improved inter-poly dielectriclayer;

FIGS. 46A, 46B and 46C illustrate an alternate method of forming an IPDlayer;

FIGS. 47A and 47B are cross-sectional views of yet another method offorming a high quality inter-poly dielectric layer;

FIGS. 48 and 49A-49D show other embodiments for formation of an improvedIPD layer;

FIG. 50A shows an anisoptric plasma etch process for IPD planarization;

FIG. 50B shows an alternate IPD planarization method using a chemicalmechanical process;

FIG. 51 is a flow diagram for an exemplary method for controllingoxidation rate;

FIG. 52 shows an improved method for forming thick oxide at the bottomof a trench using a sub-atmospheric chemical vapor deposition process;

FIG. 53 is an exemplary flow diagram of a method for forming thick oxideat the bottom of a trench using a directional Tetraethoxyorthsilicateprocess;

FIGS. 54 and 55 show another embodiment for forming thick bottom oxide;

FIGS. 56-59 show another process for forming a thick dielectric layer atthe bottom of a trench;

FIG. 60 is a simplified diagram of a MOSFET with a current sense device;

FIG. 61A is an example of a charge balance MOSFET with a planar gatestructure and isolated current sense structure;

FIG. 61B shows an example of integrating a current sense device with atrench MOSFET;

FIGS. 62A-62C show alternative embodiments for a MOSFET with seriestemperature sensing diodes;

FIGS. 63A and 63B show alternative embodiments for a MOSFET with ESDprotection;

FIGS. 64A-64D show examples of ESD protection circuits;

FIG. 65 shows an exemplary process for forming charge balanced powerdevices with lower ESR;

FIGS. 66A and 66B show a layout technique to reduce ESR;

FIG. 67 shows a DC-DC converter circuit using power switching;

FIG. 68 shows another DC-DC converter circuit using power switching;

FIG. 69 shows an exemplary driver circuit for a dual gate MOSFET;

FIG. 70A shows an alternate embodiment with separately driven gateelectrodes;

FIG. 70B shows a timing diagram illustrating the operation of thecircuit of FIG. 70A;

FIG. 71 is a simplified cross-sectional view of a molded package; and

FIG. 72 is a simplified cross-sectional view of an unmolded package.

DETAILED DESCRIPTION OF THE INVENTION

The power switch can be implemented by any one of power MOSFET, IGBT,various types of thyristors and the like. Many of the novel techniquespresented herein are described in the context of the power MOSFET forillustrative purposes. It is to be understood however that the variousembodiments of the invention described herein are not limited to thepower MOSFET and can apply to many of the other types of power switchtechnologies, including, for example, IGBTs and other types of bipolarswitches and various types of thyristors, as well as diodes. Further,for the purposes of illustration, the various embodiments of theinvention are shown to include specific p and n type regions. It isunderstood by those skilled in the art that the teachings herein areequally applicable to devices in which the conductivities of the variousregions are reversed.

Referring to FIG. 1, there is shown a cross-sectional view of a portionof an exemplary n-type trench power MOSFET 100. As with all otherfigures described herein, it is to be understood that the relativedimensions and sizes of various elements and components depicted in thefigures do not exactly reflect actual dimensions and are forillustrative purposes only. Trench MOSFET 100 includes a gate electrodethat is formed inside trenches 102 that extend from the top surface ofthe substrate through a p-type well or body region 104, terminating inan n-type drift or epitaxial region 106. Trenches 102 are lined withthin dielectric layers 108 and are substantially filled with conductivematerial 110 such as doped polysilicon. N-type source regions 112 areformed inside body region 104 adjacent to trenches 102. A drain terminalfor MOSFET 100 is formed at the backside of the substrate connecting toa heavily-doped n+ substrate region 114. The structure shown in FIG. 1is repeated many times on a common substrate made of, for example,silicon, to form an array of transistors. The array may be configured invarious cellular or striped architectures known in this art. When thetransistor is turned on, a conducting channel is formed verticallybetween source regions 112 and drift region 106 along the walls of gatetrenches 102.

Because of its vertical gate structure, MOSFET 100 enables a higherpacking density when compared with a planar gate device, and the higherpacking density translates to relatively lower on-resistance. To improvethe breakdown voltage performance of this transistor, p+ heavy bodyregion 118 is formed inside p− well 104 such that at the interfacebetween p+ heavy body 118 and p− well 104 an abrupt junction is formed.By controlling the depth of p+ heavy body 118 relative to the trenchdepth and the depth of the well, electric fields that are generated whenvoltage is applied to the transistor are moved away from the trenches.This increases avalanche current handling capability of the transistor.Variations on this improved structure and processes for forming thetransistor, and in particular the abrupt junction, are described ingreater detail in commonly owned U.S. Pat. No. 6,429,481, to Mo et al.,which is hereby incorporated by reference in its entirety.

Although vertical trench MOSFET 100 exhibits good on-resistance andimproved ruggedness, it has a relatively high input capacitance. Theinput capacitance for trench MOSFET 100 has two components:gate-to-source capacitance Cgs and gate-to-drain capacitance Cgd. Thegate-to-source capacitance Cgs results from the overlap between gateconductive material 110 and source regions 112 near the top of thetrench. The capacitance formed between the gate and the inverted channelin the body also contributes to Cgs since in typical power switchingapplications the body and source electrodes of the transistor areshorted together. The gate-to-drain capacitance Cgd results from theoverlap between gate conductive material 110 at the bottom of eachtrench and drift region 106 which connects to the drain. Thegate-to-drain capacitance Cgd, or Miller capacitance, limits thetransistor V_(DS) transition time. Therefore, higher Cgs and Cgd resultsin appreciable switching losses. These switching losses are becomingincreasingly important as power management applications move towardhigher switching frequencies.

One way to reduce the gate-to-source capacitance Cgs is to reduce thechannel length of the transistor. A shorter channel length directlyreduces the gate-to-channel component of Cgs. A shorter channel lengthis also directly proportional to R_(DSon) and enables obtaining the samedevice current capacity with fewer gate trenches. This reduces both Cgsand Cgd by reducing the amount of gate-to-source and gate-to-drainoverlap. A shorter channel length, however, renders the devicevulnerable to punch through when the depletion layer formed as a resultof the reverse-biased body-drain junction pushes deep into the bodyregion and approaches the source regions. Decreasing the dopingconcentration of the drift region so that it sustains more of thedepletion layer has the undesirable effect of increasing theon-resistance R_(DSon) of the transistor.

An improvement to the transistor structure that allows a reduction inchannel length and is also effective in addressing the above drawbacksuses additional “shield” trenches that are laterally spaced from gatetrenches. Referring to FIG. 2A, there is shown an exemplary embodimentof a dual trench MOSFET 200. The terminology “dual trench” refers to thetransistor having two different types of trenches as opposed to thetotal number of similar trenches. In addition to the structural featuresthat are common to the MOSFET of FIG. 1, dual trench MOSFET 200 includesshield trenches 220 that are interposed between adjacent gate trenches202. In the exemplary embodiment shown in FIG. 2A, shield trenches 220extend from the surface through p+ region 218, body region 204 and intodrift region 206 well below the depth of gate trenches 202. Trenches 220are lined with a dielectric material 222 and are substantially filledwith conductive material 224 such as doped polysilicon. A metal layer216 electrically connects conductive material 224 inside trenches 220with the n+ source regions 212 and p+ heavy body regions 218. In thisembodiment, trenches 220 can therefore be referred to as source shieldtrenches. An example of this type of dual trench MOSFET, and process ofmanufacture and circuit applications for the same are described ingreater detail in commonly-assigned, U.S. patent application Ser. No.10/209,110, entitled “Dual Trench Power MOSFET,” by Steven Sapp, whichis hereby incorporated by reference in its entirety.

The impact of deeper source shield trenches 220 is to push the depletionlayer formed as a result of the reverse-biased body-drain junctiondeeper into drift region 206. Thus, a wider depletion region can resultwithout increasing the electric field. This allows the drift region tobe more highly doped without lowering the breakdown voltage. A morehighly doped drift region reduces the transistor on-resistance.Moreover, the reduced electric field near the body-drain junction allowsthe channel length to be substantially reduced which further reduces theon-resistance of the transistor and substantially reduces thegate-to-source capacitance Cgs. Also, as compared to the MOSFET of FIG.1, the dual trench MOSFET enables obtaining the same transistor currentcapacity with far fewer gate trenches. This significantly reduces thegate-to-source and gate-to-drain overlap capacitances. Note that in theexemplary embodiment shown in FIG. 2A, gate trench conductive layer 210is buried inside the trench eliminating the need for the interlayerdielectric dome that is present above trenches 102 in MOSFET 100 shownin FIG. 1. Also, the use of source shield trenches as taught herein isnot limited to trench gated MOSFETs and similar advantages are obtainedwhen source shield trenches are employed in planar MOSFETs where thegate is formed horizontally on the top surface of the substrate. Anexemplary embodiment for a planar gate MOSFET with source shield trenchstructure is shown in FIG. 2B.

To further reduce the input capacitance, additional structuralimprovements can be made that focus on reducing the gate-to-draincapacitance Cgd. As discussed above, the gate-to-drain capacitance Cgdis caused by the overlap between the gate and the drift region at thebottom of the trench. One method of reducing this capacitance increasesthe thickness of the gate dielectric layer at the bottom of the trench.Referring back to FIG. 2A, gate trenches 202 are depicted as having athicker dielectric layer 226 at the bottom of the trench where there isoverlap with drift region 206 (the transistor drain terminal) ascompared to dielectric layer along the sidewalls of the gate trench.This reduces the gate-to-drain capacitance Cgd without degrading theforward conduction of the transistor. Creating a thicker dielectriclayer at the bottom of the gate trench can be accomplished in a numberof different ways. One exemplary process for creating the thickerdielectric layer is described in commonly-owned U.S. Pat. No. 6,437,386to Hurst et al. which is hereby incorporated by reference in itsentirety. Other processes for forming a thick dielectric layer at thebottom of a trench are described further below in connection with FIGS.56 to 59. Another way to minimize the gate-to-drain capacitance is toinclude a centrally disposed second dielectric core inside the trenchthat extends upwardly from the dielectric liner on the trench floor. Inone embodiment, the second dielectric core may extend all the way up tocontact the dielectric layer above the trench conductive material 210.An example of this embodiment, and variations thereof, are described ingreater detail in commonly-owned U.S. Pat. No. 6,573,560 to Shenoy.

Another technique for reducing the gate-to-drain capacitance Cgdinvolves shielding the gate using one or more biased electrodes.According to this embodiment, inside the gate trench and below theconductive material that forms the gate electrode, one or moreelectrodes are formed to shield the gate from the drift region, therebysubstantially reducing the gate-to-drain overlap capacitance. Referringto FIG. 3A, there is shown part of an exemplary embodiment of a shieldedgate trench MOSFET 300A. Trenches 302 in MOSFET 300A include a gateelectrode 310 and, in this example, two additional electrodes 311 a and311 b under gate electrode 310. Electrodes 311 a and 311 b shield gateelectrode 310 from having any substantial overlap with drift region 306almost eliminating the gate-to-drain overlap capacitance. Shieldelectrodes 311 a and 311 b can be independently biased at optimalpotential. In one embodiment, one of shield electrodes 311 a or 311 bmay be biased at the same potential as the source terminal. Similar tothe dual trench structure, the biasing of the shield electrodes can alsohelp in widening of the depletion region formed at the body-drainjunction which further reduces Cgd. It is to be understood that thenumber of shield electrodes 311 can vary depending on the switchingapplication and in particular the voltage requirements of theapplication. Similarly, the size of the shield electrodes in a giventrench can vary. For example, shield electrode 311 a can be larger thanshield electrode 311 b. In one embodiment, the smallest shield electrodeis the closest to the bottom of the trench and the remaining shieldelectrodes gradually increase in size as they near the gate electrode.Independently biased electrodes inside trenches can also be used forvertical charge control purposes to improve smaller forward voltage lossand higher blocking capability. This aspect of the transistor structure,which will be described further below in connection with higher voltagedevices, is also described in greater detail in commonly-assigned U.S.patent application Ser. No. 09/981,583, entitled “SemiconductorStructure with Improved Smaller Forward Voltage Loss and Higher BlockingCapability,” by Kocon, which is hereby incorporated by reference in itsentirety.

FIG. 3B illustrates an alternative embodiment for a shielded gate trenchMOSFET 300B that combines the dual trench structure of FIG. 2A with theshielded gate structure of FIG. 3A. In the exemplary embodiment shown inFIG. 3B, gate trench 301 includes gate poly 310 above shield poly 311similar to trench 302 of MOSFET 300A. MOSFET 300B, however, includesnon-gate trenches 301 that may be deeper than gate trenches 302 forvertical charge control purposes. While the charge control trenches 301may have a single layer of conductive material (e.g., polysilicon)connecting to the source metal at the top of the trench, as in FIG. 2A,the embodiment shown in FIG. 3B uses multiple stacked poly electrodes313 that can be independently biased. The number of electrodes 313stacked in a trench can vary depending on the application requirements,as can the sizes of electrodes 313 as shown in FIG. 3B. The electrodescan be independently biased or tied together electrically. Also thenumber of charge control trenches inside a device will depend on theapplication.

Yet another technique for improving the switching speed of the powerMOSFET reduces the gate-to-drain capacitance Cgd by employing a dualgate structure. According to this embodiment, the gate structure insidethe trench is split into two segments: a first segment that performs theconventional gate function receiving the switching signal, and a secondsegment that shields the first gate segment from the drift (drain)region and can be independently biased. This dramatically reduces thegate-to-drain capacitance of the MOSFET. FIG. 4A is a simplified partialdiagram of an exemplary embodiment of a dual gate trench MOSFET 400A. Asdepicted in FIG. 4A, the gate of MOSFET 400A has two segments G1 and G2.Unlike the shielding electrodes (311 a and 311 b) in MOSFET 300A of FIG.3A, the conductive material that forms G2 in MOSFET 400A has an overlapregion 401 with the channel and therefore acts as a gate terminal. Thissecondary gate terminal G2, however, is biased independently of theprimary gate terminal G1 and does not receive the same signal thatdrives the switching transistor. Instead, in one embodiment, G2 isbiased at a constant potential just above the threshold voltage of theMOSFET to invert the channel in overlap region 401. This will ensurethat a continuous channel is formed when transitioning from secondarygate G2 to primary gate G1. Also, Cgd is reduced because the potentialat G2 is higher than the source potential, and the charge transfer awayfrom the drift region and into the secondary gate G2 further contributesto the reduction in Cgd. In another embodiment, instead of a constantpotential, secondary gate G2 can be biased to a potential above thethreshold voltage just prior to a switching event. In other embodiments,the potential at G2 can be made variable and optimally adjusted tominimize any fringing portion of the gate-to-drain capacitance Cgd. Thedual gate structure can be employed in MOSFETs with planar gatestructure as well as other types of trench gate power devices includingIGBTs and the like. Variations on the dual gate trench MOS gated devicesand processes for manufacturing such devices are described in greaterdetail in commonly-assigned U.S. patent application Ser. No. 10/640,742,entitled “Improved MOS Gating Method for Reduced Miller Capacitance andSwitching Losses,” by Kocon et al., which is hereby incorporated byreference in its entirety.

Another embodiment for an improved power MOSFET is shown in FIG. 4B,wherein an exemplary MOSFET 400B combines a planar dual gate structurewith trenched electrodes for vertical charge control. Primary andsecondary gate terminals G1 and G2 function in a similar fashion as thetrenched dual gate structure of FIG. 4A, while deep trenches 420 providean electrode in the drift region to spread charge and increase breakdownvoltage of the device. In the embodiment shown, shield or secondary gateG2 overlaps the upper portion of primary gate G1 and extends over p well404 and drift region 406. In an alternative embodiment, primary gate G1extends over shield/secondary gate G2.

The various techniques described thus far such as gate shielding andtrenched electrodes for vertical charge control can be combined toobtain power devices, including lateral and vertical MOSFETs, IGBTs,diodes and the like, whose performance characteristics are optimized fora given application. For example, the trenched dual gate structure shownin FIG. 4A can be advantageously combined with vertical charge controltrench structures of the types shown in FIG. 3B or 4B. Such a devicewould include an active trench with dual gate structure as shown in FIG.4A as well as deeper charge control trenches that are eithersubstantially filled by a single layer of conductive material (as intrenches 420 in FIG. 4B) or by multiple stacked conductive electrodes(as in trenches 301 in FIG. 3B). For lateral devices where the drainterminal is located on the same surface of the substrate as the sourceterminal (i.e., current flows laterally), the charge control electrodeswould be laterally disposed forming field plates, instead of beingstacked in vertical trenches. The orientation of the charge controlelectrodes is generally parallel to the direction of current flow in thedrift region.

In one embodiment, the dual gate and shielded gate techniques arecombined inside the same trench to provide switching speed and blockingvoltage enhancements. FIG. 4C shows a MOSFET 400C wherein trench 402Cincludes a primary gate G1, a secondary gate G2 and a shield layer 411stacked in a single trench as shown. Trench 402C can be made as deep andmay include as many shield layers 411 as the application demands. Usingthe same trench for both charge balance and shielding electrodes enableshigher density, since it eliminates the need for two trenches andcombines it into one. It also enables more current spreading andimproves device on-resistance. It is to be understood, however, thatembodiments combining active trenches having the shielded dual gatestructure of the type shown in FIG. 4C with separate charge balancingtrenches of the various types described herein are also possible.

The devices described thus far employ combinations of shielded gate,dual gate and other techniques to reduce parasitic capacitance. Due tofringing effects, however, these techniques do not fully minimize thegate-to-drain capacitance Cgd. Referring to FIG. 4D, there is shown apartial cross-sectional view of an exemplary embodiment of MOSFET 400Dwith deep body design. According to this embodiment, the body structureis formed by a trench 418 that is etched through the center of the mesaformed between gate trenches 402, and extends as deep or deeper thangate trench 402. Body trench 418 is filled with source metal as shown.The source metal layer may include a thin refractory metal at themetal-diffusion boundary (not shown). In this embodiment, the bodystructure further includes a p+ body implant 419 that substantiallysurrounds body trench 418. P+ implant layer 419 enables additionalshielding to alter the potential distribution inside the deviceespecially close to the gate electrode. In an alternate embodiment shownin FIG. 4E, body trench 418 is substantially filled with epitaxialmaterial using, for example, selective epitaxial growth (SEG)deposition. Alternatively, body trench 418E is substantially filled withdoped polysilicon. In either of these two embodiments, instead ofimplanting p+ shield junction 419, subsequent temperature treatment willdiffuse dopants from the filled body into the silicon to form p+ shieldjunction 419. A number of variations for trenched body structure andformation are described in greater detail in commonly-assigned U.S. Pat.Nos. 6,437,399 and 6,110,799, both to Huang, which are herebyincorporated by reference in their entirety.

In both embodiments shown in FIGS. 4D and 4E, the distance L betweengate trench 402 and body trench 418, as well as the relative depths ofthe two trenches are controlled to minimize fringing gate-to-draincapacitance. In the embodiments using SEG or poly filled body trenches,the spacing between the outer edges of the layer 419 and the wall of thegate trench can be adjusted by varying the doping concentration of theSEG or poly inside body trench 418. FIGS. 4F and 4G illustrate theimpact of the trenched deep body on the distribution of the potentiallines inside the device near the gate electrode. For illustrativepurposes, FIGS. 4F and 4G use MOSFETs with shielded gate structures.FIG. 4F shows the potential lines for a reverse biased shielded gateMOSFET 400F with trenched deep body 418, and FIG. 4G shows the potentiallines for a reverse biased shielded gate MOSFET 400G with a shallow bodystructure. The contour lines in each device show potential distributioninside the device when reverse biased (i.e., blocking off-state). Thewhite line shows the well junction and also defines the bottom of thechannel located next to the gate electrode. As can be seen from thediagrams, there is a lower potential and lower electric field imposed onthe channel and surrounding gate electrode for the trenched deep bodyMOSFET 400F of FIG. 4F. This decreased potential enables a reducedchannel length which reduces the total gate charge for the device. Forexample, the depth of gate trench 402 can be reduced to below, e.g., 0.5um, and can be made shallower than body trench 418 with the spacing Lbeing about 0.5 um or smaller. In one exemplary embodiment, the spacingL is less than 0.3 um. Another advantage of this embodiment is thereduction in the gate-drain charge Qgd and Miller capacitance Cgd. Thelower the value of these parameters, the faster the device is able toswitch. This improvement is realized through the reduction of potentialthat is present next to the gate electrode. The improved structure hasmuch lower potential that will be switched and the induced capacitivecurrent in the gate is much lower. This in turn enables the gate toswitch faster.

The trenched deep body structure as described in connection with FIGS.4D and 4E can be combined with other charge balancing techniques such asshielded gate or dual gate structures, to further improve the switchingspeed, on-resistance, and blocking capability of the device.

The improvements provided by the above power devices and variationsthereof have yielded robust switching elements for relatively lowervoltage power electronic applications. Low voltage as used herein refersto a voltage range from, for example, about 30V-40V and below, thoughthis range may vary depending on the particular application.Applications requiring blocking voltages substantially above this rangenecessitate some type of structural modification to the powertransistor. Typically, the doping concentration in the drift region ofthe power transistor is reduced in order for the device to sustainhigher voltages during the blocking state. A more lightly doped driftregion, however, results in an increase in the transistor on-resistanceR_(DSon). The higher resistivity directly increases the power loss ofthe switch. The power loss has become more significant as recentadvances in semiconductor manufacturing have further increased thepacking density of power devices.

Attempts have been made to improve the device on-resistance and powerloss while maintaining high blocking voltage. Many of these attemptsemploy various vertical charge control techniques to create a largelyflat electric field vertically in the semiconductor device. A number ofdevice structures of this type have been proposed including the lateraldepletion device disclosed in commonly-owned U.S. Pat. No. 6,713,813,entitled “Field Effect Transistor Having a Lateral Depletion Structure,”by Marchant, and the devices described in commonly-owned U.S. Pat. No.6,376,878, to Kocon, both of which are hereby incorporated by referencein their entirety.

FIG. 5A shows a cross-sectional view of a portion of an exemplary powerMOSFET 500A with a planar gate structure. MOSFET 500A appears to havesimilar structure to that of planar MOSFET 200B of FIG. 2B, but itdiffers from that device in two significant respects. Instead of fillingtrenches 520 with conductive material, these trenches are filled withdielectric material such as silicon dioxide, and the device furtherincludes discontinuous floating p-type regions 524 spaced adjacent theouter sidewalls of trenches 520. As described in connection with thedual trench MOSFET of FIG. 2A, the conductive material (e.g.,polysilicon) in source trenches 202 help improve the cell breakdownvoltage by pushing the depletion region deeper into the drift region.Eliminating the conductive material from these trenches would thusresult in lowering the breakdown voltage unless other means of reducingthe electric field are employed. Floating p regions 524 serve to reducethe electric field.

Referring to MOSFET 500A shown in FIG. 5A, as the electric fieldincreases when the drain voltage is increased, floating p regions 524acquire a corresponding potential determined by their position in thespace charge region. The floating potential of these p regions 524causes the electric field to spread deeper into the drift regionresulting in a more uniform field throughout the depth of the mesaregion in between trenches 520. As a result, the breakdown voltage ofthe transistor is increased. The advantage of replacing the conductivematerial in the trenches with insulating material is that a greaterportion of the space charge region appears across an insulator ratherthan the drift region which could be silicon. Because the permittivityof an insulator is lower than that of, e.g. silicon, and because thearea of the depletion region in the trench is reduced, the outputcapacitance of the device is significantly reduced. This furtherenhances the switching characteristics of the transistor. The depth ofdielectric-filled trenches 520 depends on the voltage requirements; thedeeper the trenches the higher the blocking voltage. An added advantageof the vertical charge control technique is that it allows thetransistor cells to be laterally displaced for thermal isolation withoutappreciable added capacitance. In an alternative embodiment, instead ofthe floating p regions, p-type layers line the exterior sidewalls of thedielectrically-filled trenches to achieve similar vertical chargebalancing. A simplified and partial cross-sectional view of thisembodiment is shown in FIG. 5B, where the exterior sidewalls of trenches520 are covered by a p-type layer or liner 526. In the exemplaryembodiment shown in FIG. 5B the gate is also trenched, which furtherimproves the device transconductance. Other embodiments for improvedpower devices employing variations of this technique are described ingreater detail in commonly-assigned U.S. patent application Ser. No.10/200,056 (Attorney Docket No. 18865-0097/17732-55280), entitled“Vertical Charge Control Semiconductor Device with Low OutputCapacitance,” by Sapp et al., which is hereby incorporated by referencein its entirety.

As described above, trench MOSFET 500B of FIG. 5B exhibits reducedoutput capacitance and improved breakdown voltage. However, because theactive trench (gate trench 502) is positioned between dielectric-filledcharge control trenches 520, the channel width of MOSFET 500B is not aslarge as conventional trench MOSFET structures. This may result in ahigher on-resistance R_(DSon). Referring to FIG. 5C, there is shown analternative embodiment for a trench MOSFET 500C with vertical chargecontrol that eliminates the secondary charge control trenches. Trenches502C in MOSFET 500C include gate poly 510 and a dielectric-filled lowerportion that extends deep into drift region 506. In one embodiment,trenches 502C extend to a depth below about half the depth of driftregion 506. A p-type liner 526C surrounds the exterior walls along thelower portion of each trench as shown. This single-trench structureeliminates the secondary charge control trench, allowing for increasedchannel width and lower R_(DSon). The lower portion of deeper trench502C that is surrounded by a p-type liner 526C on its exterior wallssupports a major portion of the electric field in order to reduce outputcapacitance and gate-to-drain capacitance. In an alternative embodiment,p-type liner 526C is made into a plurality of discontinuous regionsalong the sides and the bottom of trench 502C. Other embodiments arepossible by combining the single trench charge control structure withshielded gate or dual gate techniques described above, to further reducedevice parasitic capacitance.

Referring to FIG. 6, there is shown a simplified cross-sectional view ofa power MOSFET 600 that is suitable for higher voltage applications thatalso require faster switching. MOSFET 600 combines vertical chargecontrol to improve breakdown voltage, with shielded gate structure thatimproves switching speed. As shown in FIG. 6, a shield electrode 611 ispositioned inside gate trench 602 between gate conductive material 610and the bottom of the trench. Electrode 611 shields the gate of thetransistor from underlying drain region (drift region 606) whichsignificantly reduces the gate-to-drain capacitance of the transistorand thus increases its maximum switching frequency. Dielectric-filledtrenches 620 with p doped liners 626 help create a largely flat electricfield vertically to improve the breakdown voltage of the device. Whilein operation, the combination of dielectric-filled trenches 620 withp-type liner 626, and the shielded gate structure reduces the parasiticcapacitance and helps deplete the n drift region which disperses theelectric field concentrating on the edge portion of the gate electrode.Devices of this type can be used in RF amplifier or in high frequencyswitching applications.

FIG. 7 depicts an alternative embodiment for another power MOSFETsuitable for higher voltage, higher frequency applications. In thesimplified example shown in FIG. 7, MOSFET 700 combines vertical chargecontrol to improve breakdown voltage with dual gate structure thatimproves switching speed. Similar to the device shown in FIG. 6,vertical charge control is implemented by the use of dielectric-filledtrenches 720 with p-doped liners 726. Reduction in parasitic capacitanceis achieved by the use of a dual gate structure whereby a primary gateelectrode G1 is shielded from the drain (n− drift region 706) by asecondary gate electrode G2. Secondary gate electrode G2 can be eithercontinuously biased or only biased prior to a switching event in orderinvert the channel in region 701 to ensure an uninterrupted flow ofcurrent through a continuous channel when the device is turned on.

In another embodiment, the shielded vertical charge control MOSFET alsoemploys the doped sidewall dielectric-filed trenches to implement anintegrated Schottky diode. FIG. 8 shows one example of a shielded gateMOSFET 800 according to this embodiment. In this example, electrode 811in the lower part of trench 802 shields gate electrode 810 from driftregion 806 to reduce parasitic gate-to-drain capacitance.Dielectric-filled trenches 820 with p doped liners on their exteriorsidewalls provide for vertical charge control. A Schottky diode 828 isformed between two trenches 820A and 820B that form a mesa of width W.This Schottky diode structure is interspersed throughout the trenchMOSFET cell array to enhance the performance characteristics of theMOSFET switch. The forward voltage drop is reduced by taking advantageof the low barrier height of Schottky structure 828. In addition, thisdiode will have an inherent reverse recovery speed advantage compared tothe normal PN junction of the vertical power MOSFET. By doping of thesidewalls of dielectric-filled trenches 820 with, e.g., Boron, sidewallleakage path due to phosphorus segregation is eliminated. Features ofthe trench process can be used to optimize the performance of Schottkydiode 828. In one embodiment, for example, the width W is adjusted suchthat depletion in the drift region of Schottky structure 828 isinfluenced and controlled by the adjacent PN junction to increase thereverse voltage capability of Schottky diode 828. An example of amonolithically integrated trench MOSFET and Schottky diode can be foundin commonly-assigned U.S. Pat. No. 6,351,018 to Sapp, which is herebyincorporated by reference in its entirety.

It is to be understood that a Schottky diode formed betweendielectric-filled trenches of the type depicted in FIG. 8 can beintegrated with a variety of different types of MOSFETs, includingMOSFETs with a planar gate structure, trench gate MOSFETs without anyshielding electrode with or without thick dielectric at the bottom ofthe trench, etc. An exemplary embodiment for a dual gate trench MOSFETwith integrated Schottky diode is shown in FIG. 9A. MOSFET 900A includesgate trench 902 wherein a primary gate G1 is formed above a secondarygate G2 to reduce parasitic capacitance and increase switchingfrequency. MOSFET 900A also includes dielectric-filled trenches 920 withp doped liners 926 formed along their exterior sidewalls for verticalcharge control to enhance the device blocking voltage. One method offorming the liners for many of the embodiments described above (e.g.,those shown in FIGS. 5B, 6, 7, 8 and 9A) uses a plasma doping process.Schottky diode 928A is formed between two adjacent dielectric-filledtrenches 920A and 920B as shown. In another variation, a monolithicallyintegrated Schottky diode and trench MOSFET is formed without thedielectric-filled trenches. FIG. 9B is a cross-sectional view of anexemplary device 900B according to this embodiment. MOSFET 900B includesactive trenches 902B each having electrodes 911 buried under a gateelectrode 910. A Schottky diode 928B is formed between two trenches 902Land 902R as shown. The charge balancing effect of biased electrodes 911allows for increasing the doping concentration of the drift regionwithout compromising the reverse blocking voltage. Higher dopingconcentration in the drift region in turn reduces the forward voltagedrop for this structure. As in previously described trench MOSFETs withburied electrodes, the depth of each trench as well as the number of theburied electrodes may vary. In one variation shown in FIG. 9C, trench902C has only one buried electrode 911 and gate electrodes 910S inSchottky cell 928C connect to the source electrode as shown. The gate ofthe Schottky diode can alternatively connect to the gate terminal of theMOSFET. FIGS. 9D, 9E and 9F show exemplary layout variations forSchottky diode interspersed within the active cell array of MOSFET.FIGS. 9D and 9E show single mesa Schottky and double mesa Schottkylayouts, respectively, while FIG. 9F shows a layout wherein Schottkyregions are perpendicular to MOSFET trenches. These and other variationsof an integrated Schottky diode, including alternative multiples ofSchottky to MOSFET regions, can be combined with any of the transistorstructures described herein.

In another embodiment, the voltage blocking capability of a power deviceis enhanced by use of one or more diode structures in series, buriedinside a trench lined with dielectric, and arranged parallel to thecurrent flow in the device drift region. FIG. 10 provides a simplifiedcross-sectional view of an exemplary trench MOSFET 1000 according tothis embodiment. Diode trenches 1020 are disposed on either sides of agate trench 1002, extending well into drift region 1006. Diode trenches1020 include one or more diode structures made up of oppositeconductivity type regions 1023 and 1025 that form one or more PNjunctions inside the trench. In one embodiment, trench 1020 includes asingle region having a polarity that is opposite that of the driftregion such that a single PN junction is formed at the interface withthe drift region. P-type and n-type doped polysilicon or silicon may beused to form regions 1023 and 1025, respectively. Other types ofmaterial such as silicon carbide, gallium arsenide, silicon germanium,etc. could also be used to form regions 1023 and 1025. A thin dielectriclayer 1021 extending along the trench inner sidewalls insulates thediode in the trench from drift region 1006. As shown, there is nodielectric layer along the bottom of trenches 1020, thus allowing thebottom region 1027 to be in electrical contact with the underlyingsubstrate. In one embodiment, similar considerations to those dictatingthe design and manufacture of the gate oxide 1008 are applied indesigning and forming dielectric layer 1021. For example, the thicknessof dielectric layer 1021 is determined by such factors as the voltage itis required to sustain and the extend to which the electric field in thediode trench is to be induced in the drift region (i.e., the extent ofcoupling through the dielectric layer).

In operation, when MOSFET 1000 is biased in its blocking state, PNjunctions inside diode trench 1020 are reverse biased with the peakelectric field occurring at each diode junction. Through dielectriclayer 1021, the electric field in the diode trench induces acorresponding electric field in drift region 1006. The induced field ismanifested in the drift region in the form of an up-swing spike and ageneral increase in the electric field curve in the drift region. Thisincrease in the electric field results in a larger area under theelectric field curve which in turn results in a higher breakdownvoltage. Variations on this embodiment are described in greater detailin commonly-assigned U.S. patent application Ser. No. 10/288,982(Attorney Docket No. 18865-117/17732-66560), entitled “Drift RegionHigher Blocking Lower Forward Voltage Drop Semiconductor Structure,” byKocon et al., which is hereby incorporated by reference in its entirety.

Other embodiments for power devices that combine trenched diodes forcharge balancing with techniques to reduce parasitic capacitance such asshielded gate or dual gate structures are possible. FIG. 11 shows oneexample of a MOSFET 1100 according to one such embodiment. MOSFET 1100uses a shield electrode 1111 inside active trench 1102 under gateelectrode 1110, to reduce gate-to-drain capacitance Cgd for thetransistor as described above in connection with, for example, MOSFET300A in FIG. 3A. A different number of PN junctions are employed inMOSFET 1100 as compared to MOSFET 1000. FIG. 12 is a cross-sectionalview of a MOSFET 1200 that combines the dual gate technique with thetrenched diode structure. Active trench 1202 in MOSFET 1200 includes aprimary gate G1 and a secondary gate G2 and operates in the same manneras the active trenches in the dual gate MOSFET described in connectionwith FIG. 4B. Diode trenches 1220 provide charge balancing to increasethe device blocking voltage while the dual gate active trench structureimproves the device switching speed.

Yet another embodiment combines the trenched diode charge balancingtechnique with integrated Schottky diode in a planar gate MOSFET 1300 asshown in FIG. 13. Similar advantages can be obtained by the integrationof Schottky diode 1328 with the MOSFET as described in connection withthe embodiments of FIGS. 8 and 9. In this embodiment, a planar gatestructure is shown for illustrative purposes, and those skilled in theart will appreciate that the combination of an integrated Schottky diodeand trenched diode structure can be employed in a MOSFET having any ofthe other types of gate structures including trench gate, dual gate andshielded gate. Any one of the resulting embodiments can also be combinedwith the trenched body technique to further minimize the fringingparasitic capacitance, as described in connection with MOSFET 400D or400E of FIGS. 4D and 4E. Other variations and equivalents are possible.For example, the number of regions of opposite conductivity inside thediode trenches may vary as can the depth of the diode trenches. Thepolarities of the opposite conductivity regions may be reversed as canthe polarity of the MOSFET. Also, any of the PN regions (923,925 or1023,1025, etc.) may be independently biased if desired by, for example,extending the respective regions along the third dimension and then upto the silicon surface where electrical contact can be made to them.Further, multiple diode trenches may be used as demanded by the size ofthe device and the voltage requirements of the application, and thespacing and arrangement of the diode trenches can be implemented invarious stripe or cellular designs.

In another embodiment, a class of accumulation-mode transistors isprovided that employs various charge balancing techniques for smallerforward voltage loss and higher blocking capability. In a typicalaccumulation-mode transistor there is no blocking junction and thedevice is turned off by lightly inverting the channel region next to thegate terminal to pinch off the current flow. When the transistor isturned on by applying a gate bias, an accumulation layer rather than aninversion layer is formed in the channel region. Since there is noinversion channel forming, channel resistance is minimized. In addition,there is no PN body diode in an accumulation-mode transistor whichminimizes the losses that are otherwise incurred in certain circuitapplications such as synchronous rectifiers. The drawback ofconventional accumulation-mode devices is that the drift region has tobe lightly doped to support a reverse bias voltage when the device is inblocking mode. A more lightly doped drift region translates to higheron-resistance. Embodiments described herein overcome this limitation byemploying various charge balancing techniques in an accumulation-modedevice.

Referring to FIG. 14, there is shown a simplified embodiment of anexemplary accumulation-mode transistor 1400 with alternate conductivityregions arranged in parallel to the current flow. In this example,transistor 1400 is an n-channel transistor with a gate terminal formedinside trenches 1402, an n-type channel region 1412 that is formedbetween trenches, a drift region 1406 that includes opposite polaritycolumnar n-type and p-type sections 1403 and 1405, and an n-type drainregion 1414. Unlike enhancement-mode transistors, accumulation-modetransistor 1400 does not include a blocking (p-type in this example)well or body region inside which the channel is formed. Instead, aconducting channel is formed when an accumulation layer is formed inregion 1412. Transistor 1400 is normally on or off depending on dopingconcentration of region 1412 and doping type of the gate electrode. Itis turned off when n-type region 1412 is entirely depleted and lightlyinverted. The doping concentrations in opposite polarity regions 1403and 1405 are adjusted to maximize charge spreading, which enables thetransistor to support higher voltages. The use of columnar oppositepolarity regions parallel to current flow flattens the electric fielddistribution by not allowing it to decrease linearly away from thejunction formed between regions 1412 and 1406. The charge spreadingeffect of this structure allows the use of a more highly doped driftregion which reduces transistor on-resistance. The doping concentrationof the various regions may vary; for example, n-type regions 1412 and1403 may have the same or different doping concentrations. Those skilledin the art appreciate that an improved p-channel transistor can beobtained by reversing the polarities of the various regions of thedevice shown in FIG. 14. Other variations of the columnar oppositepolarity regions inside the drift region are described in greater detailin connection with ultra-high voltage devices described further below.

FIG. 15 is a simplified diagram of another accumulation-mode device 1500with trenched electrodes for charge spreading purposes. All regions1512, 1506 and 1514 are of the same conductivity type, in this example,n-type. For a normally off device, gate polysilicon 1510 is made p-type.The doping concentration of region 1512 is adjusted to form a depletedblocking junction under no bias conditions. Inside each trench 1502, oneor more buried electrodes 1511 are formed under gate electrode 1510, allsurrounded by dielectric material 1508. As described in connection withenhancement-mode MOSFET 300A of FIG. 3A, buried electrodes 1511 act asfield plates and can be biased, if desired, to a potential thatoptimizes their charge spreading function. Since charge spreading can becontrolled by independently biasing buried electrodes 1511, the maximumelectric field can be increased significantly. Similar to the buriedelectrodes employed in MOSFET 300A, different variations of thestructure are possible. For example, the depth of trench 1502 and thesize and number of buried electrodes 1511 can vary depending on theapplication. Charge spreading electrodes can be buried inside trenchesthat are separate from active trenches that house the transistor gateelectrode, in a similar fashion to that shown for the trench structuresof MOSFET 300B in FIG. 3B. An example of such an embodiment is shown inFIG. 16. In the example shown in FIG. 16, n-type region 1612 includesmore heavily doped n+ source regions 1603 that can be optionally added.Heavily doped source regions 1603 can extend along the top edge ofn-type region 1612 as shown or can be formed as two regions adjacenttrench walls along the top edge of n-type region 1612 (not shown in thisFigure). In some embodiments, the inclusion of n+ regions 1603 maynecessitate lowering the doping concentration of n-type region 1606 inorder to ensure the transistor can properly shut off. This optionalheavily doped source region can be used in the same manner in any one ofthe accumulation transistors described herein.

Another embodiment for an improved accumulation-mode transistor employsdielectric-filled trenches with an opposite polarity exterior liner.FIG. 17 is a simplified cross-sectional view of an accumulationtransistor 1700 according to this embodiment. Dielectric-filled trenches1720 extend downward from the surface of the silicon well into driftregion 1706. Trenches 1720 are substantially filed with dielectricmaterial such as silicon dioxide. In this exemplary embodiment,transistor 1700 is an n-channel transistor with trenched gate structure.A p-type region 1726 lines the exterior walls of dielectric-filledtrenches 1720 as shown. Similar to the enhancement-mode transistors500A, 500B and 500C described in connection with FIGS. 5A, 5B and 5C,respectively, trenches 1720 reduce the output capacitance of thetransistor while p-type liner 1726 provides for charge balancing in thedrift region to increase the blocking capability of the transistor. Inan alternative embodiment shown in FIG. 18, oppositely doped liners1826N and 1826P are formed adjacent the opposite sides of adielectric-filled trench 1820. That is, a dielectric-filled trench 1820has a p-type liner 1826P extending along the exterior sidewall on oneside, and an n-type liner 1826N extending along the exterior sidewall onthe other side of the same trench. Other variations of this combinationof accumulation transistor with dielectric-filled trenches, as describedin connection with the corresponding enhancement-mode transistors, arepossible. These include, for example, an accumulation transistor with aplanar (as opposed to trenched) gate structure and floating p-typeregions instead of p-type liner 1726 as in the device shown in FIG. 5A;an accumulation transistor with a p-type liner that covers only theexterior side-walls and not the bottom of trenches 1726 as in the deviceshown in FIG. 5B; and an accumulation transistor with a single trenchstructure with a p-type liner that covers the lower portion of thetrench as in the device shown in FIG. 5C, among others.

In another embodiment, an accumulation-mode transistor employs one ormore diodes formed in series inside a trench for charge balancingpurposes. A simplified cross-sectional view of an exemplaryaccumulation-mode transistor 1900 according to this embodiment is shownin FIG. 19. Diode trenches 1920 are disposed on either side of a gatetrench 1902, extending well into drift region 1906. Diode trenches 1920include one or more diode structures made up of opposite conductivitytype regions 1923 and 1925 that form one or more PN junctions inside thetrench. P-type and n-type doped polysilicon or silicon may be used toform regions 1923 and 1925. A thin dielectric layer 1921 extending alongthe trench inner sidewalls insulates the diodes in the trench from driftregion 1906. As shown, there is no dielectric layer along the bottom oftrenches 1920, thus allowing the bottom region 1927 to be in electricalcontact with the underlying substrate. Other variations of thiscombination of accumulation transistor with trenched diodes, asdescribed in connection with the corresponding enhancement-modetransistors shown in FIGS. 10, 11, 12 and 13 and variations thereof, arepossible.

Any one of the accumulation-mode transistors described above can employa heavily doped opposite polarity region in the top (source) region.FIG. 20 is a simplified three-dimensional view of an exemplaryaccumulation-mode transistor 2000 that shows this feature in combinationwith other variations. In this embodiment, the charge balancing diodesin accumulation-mode transistor 2000 are formed inside the same trenchas the gate. Trench 2002 includes gate electrode 2010 below which n-type2023 and p-type 2025 silicon or polysilicon layers form PN junctions. Athin dielectric layer 2008 separates the diode structure from gateterminal 2002 as well as drift region 2006. Heavily doped p+ regions2118 are formed at intervals along the length of the mesa formed betweentrenches in source regions 2012, as shown. Heavily doped p+ regions 2118reduce the area of n− region 2012 and reduce device leakage. P+ regions2118 also allow for p+ contact which will improve hole current flow inavalanche and improve device robustness. Variations on an exemplaryvertical MOS-gated accumulation transistor have been discussed toillustrate the various features and advantages of this class of device.One of skill in the art appreciates that these can be implemented inother types of devices including lateral MOS-gated transistors, diodes,bipolar transistors and the like. Charge spreading electrodes can beformed either inside the same trench as the gate or inside separatetrenches. The various exemplary accumulation-mode transistors describedabove have the trenches terminating in the drift regions, but they canalso terminate in the more heavily doped substrate connecting to thedrain. The various transistors can be formed in stripe or cellulararchitecture including hexagonal or square shaped transistor cells.Other variations and combinations as described with some of the otherembodiments are possible, many of which are further described inpreviously referenced U.S. Patent Application Nos. 60/506,194 and60/588,845, both of which are incorporated herein by reference in theirentirety.

Another class of power switching devices designed for very high voltageapplications (e.g., 500V-600V and above), employs alternating verticalsections of p-doped and n-doped silicon in the epitaxial region betweenthe substrate and the well. Referring to FIG. 21, there is shown oneexample of a MOSFET 2100 that employs this type of structure. In MOSFET2100, region 2102 that is sometimes referred to as the voltagesustaining or the blocking region, comprises the alternating n-typesections 2104 and p-type sections 2106. The effect of this structure isthat when voltage is applied to the device, the depletion region spreadshorizontally into each side of sections 2104 and 2106. The entirevertical thickness of blocking layer 2102 is depleted before thehorizontal field is high enough to produce avalanche breakdown becausethe net quantity of charge in each vertical section 2104,2106 is lessthan that needed to produce the breakdown field. After the region isfully depleted horizontally, the field continues to build verticallyuntil it reaches the avalanche field of approximately 20 to 30 volts permicron. This greatly enhances the voltage blocking capability of thedevice extending the voltage range of the device to 400 volts and above.Different variations of this type of super-junction device are describedin greater detail in commonly-owned U.S. Pat. Nos. 6,081,009 and6,066,878 both to Nielson, which are hereby incorporated by reference intheir entirety.

A variation on the super-junction MOSFET 2100 uses floating p-typeislands in the n-type blocking region. The use of floating p-typeislands as opposed to the pillar approach, allows the thickness of thecharge balance layer to be reduced which reduces R_(DSon). In oneembodiment, instead of uniformly spacing the p-type islands, they arespaced apart so as to maintain the electric filed near the criticalelectric field. FIG. 22 is a simplified cross-sectional view of a MOSFET2200 that shows one example of a device according to this embodiment. Inthis example, the deeper floating p regions 2226 are spaced farther fromthe ones above. That is, the distance L3 is larger than the distance L2,and the distance L2 is larger than the distance L1. By manipulating thedistance between the floating junctions in this manner, minoritycarriers are introduced in a more granular fashion. The more granularthe sources of these carriers the lower R_(DSon) and the higherbreakdown voltage can be made. It is understood by those skilled in theart that many variations are possible. For example, the number offloating regions 2226 in the vertical direction is not limited to fouras shown, and the optimum number may vary. Also, the dopingconcentration in each floating region 2226 may vary; for example, in oneembodiment, the doping concentration in each floating region 2226decreases gradually as the region gets closer to substrate 2114.

Further, many of the techniques for reducing parasitic capacitance toenhance switching speed, including shielded gate and dual gatestructures, as described in connection with low voltage and mediumvoltage devices, can be combined with the high voltage devices describedin FIGS. 21 and 22 and variations thereof. FIG. 23 is a simplifiedcross-sectional view of a high voltage MOSFET 2300 that combines avariation of the super-junction architecture with a dual gate structure.MOSFET 2300 has a planar dual gate structure made up of gate terminalsG1 and G2 similar to, for example, the dual gate transistor shown inFIG. 4B above. Opposite polarity (p-type in this example) regions 2326are vertically disposed in n-type drift region 2306 under p-well 2308.The size and spacing of p-type regions 2326 vary in this example wherebythe more closely disposed regions 2326 nearer p-well 2308 make contactto each other while regions 2326 disposed further below are floating andsmaller in size as shown. FIG. 24 depicts yet another embodiment for ahigh voltage MOSFET 2400 that combines the super-junction technologywith shielded gate structure. MOSFET 2400 is a trench gate device with agate electrode 2410 that is shielded from drift region 2406 with ashield electrode 2411 similar to, for example, MOSFET 300A in FIG. 3A.MOSFET 2400 also includes opposite polarity floating regions 2426disposed in drift region 2406 parallel to current flow.

Termination Structures

Discrete devices of the various types described above have a breakdownvoltage limited by the cylindrical or spherical shape of the depletionregion at the edge of the die. Since this cylindrical or sphericalbreakdown voltage is typically much lower than the parallel planebreakdown voltage BVpp in the active area of the device, the edge of thedevice needs to be terminated so as to achieve a breakdown voltage forthe device that is close to the active area breakdown voltage. Differenttechniques have been developed to spread the field and voltage uniformlyover the edge termination width to achieve a breakdown voltage that isclose to BVpp. These include field plates, field rings, junctiontermination extension (JTE) and different combinations of thesetechniques. The above-referenced, commonly-owned U.S. Pat. No. 6,429,481to Mo et al. describes one example of a field termination structure thatincludes a deep junction (deeper than the well) with an overlying fieldoxide layer, surrounding the active cell array. In the case of ann-channel transistor, for example, the termination structure includes adeep p+ region that forms a PN junction with the n-type drift region.

In alternative embodiments, one or more ring-shaped trenches surroundingthe periphery of the cell array act to lessen the electric field andincrease avalanche breakdown. FIG. 25A shows a commonly-used trenchlayout for a trench transistor. Active trenches 2502 are surrounded by aring-shaped termination trench 2503. In this structure, regions 2506shown by the dotted circles at the ends of the mesas deplete faster thanother regions causing increased field in this area which reduces thebreakdown voltage under reverse bias conditions. This type of layout istherefore limited to lower voltage devices (e.g., <30V). FIGS. 25B to25F show a number of alternative embodiments for termination structureswith different trench layouts to reduce the high electric field regionsshown in FIG. 25A. As can be seen by the diagrams, in these embodimentssome or all active trenches are disconnected from the terminationtrench. The gap W_(G) between the ends of the active trenches and thetermination trench function to reduce the electric field crowding effectobserved in the structure shown in FIG. 25A. In one exemplaryembodiment, W_(G) is made approximately half the width of the mesabetween trenches. For higher voltage devices, multiple terminationtrenches as shown in FIG. 25F can be employed to further increase thebreakdown voltage of the device. Commonly-owned U.S. Pat. No. 6,683,363,entitled “Trench Structure for Semiconductor Devices,” by Challa, whichis hereby incorporated in its entirety, describes variations on some ofthese embodiments in greater detail.

FIGS. 26A through 26C depict cross-sectional views of various exemplarytrench termination structures for charge balanced trench MOSFETs. In theexemplary embodiment shown, MOSFET 2600A uses a shielded gate structurewith a shield poly electrode 2611 buried under gate poly 2610 insideactive trench 2602. In the embodiment shown in FIG. 26A, terminationtrench 2603A is lined with a relatively thick layer of dielectric(oxide) 2605A and filled with conductive material such as poly 2607A.The thickness of oxide layer 2605A, the depth of termination trench2603A and the spacing between the termination trench and the adjacentactive trench (i.e., width of the last mesa) are determined by thedevice reverse blocking voltage. In the embodiment shown in FIG. 26A,trenches are wider at the surface (T-trench structure) and a metal fieldplate 2609A is used over the termination region. In an alternativeembodiment (not shown), the field plate can be formed from polysiliconby extending poly 2607A inside termination trench 2603A above thesurface and over the termination region (to the left of the terminationtrench in FIG. 26A). Many variations are possible. For example, a p+region (not shown) under the metal contacts to silicon can be added forbetter Ohmic contact. P− well region 2604 in the last mesa adjacenttermination trench 2603A and it's respective contact can be optionallyremoved. Also, floating p-type region(s) can be added to the left oftermination trench 2603A (i.e., outside active area).

In another variation, instead of filling termination trench 2603 withpoly, a poly electrode is buried in the lower portion of the trenchinside an oxide-filled trench. This embodiment is shown in FIG. 26B,wherein approximately half of termination trench 2603B is filled withoxide 2605B with the lower half having a poly electrode 2607B buriedinside the oxide. The depth of trench 2603B and height of buried poly2607B can be varied based on the device processing. In yet anotherembodiment shown in FIG. 26C, a termination trench 2603C issubstantially filled with dielectric with no conductive material buriedtherein. For all three embodiments shown in FIGS. 26A, B and C, thewidth of the last mesa separating the termination trench from the lastactive trench may be different than the width of a typical mesa formedbetween two active trenches, and can be adjusted to achieve optimalcharge balancing in the termination region. All variations describedabove in connection with the structure shown in FIG. 26A can apply tothose shown in FIGS. 26B and 26C. Further, those skilled in the artappreciate that while the termination structures have been describedherein for a shielded gate device, similar structures can be implementedas termination regions for all the various trench based devicesdescribed above.

For lower voltage devices the corner designs for the trench terminationring may not be critical. However, for higher voltage devices therounding of the corners of the termination ring with a larger radius ofcurvature may be desirable. The higher the device voltage requirements,the larger may be the radius of curvature at the corners of terminationtrench. Also the number of termination rings can be increased as thedevice voltage increases. FIG. 27 shows an exemplary device with twotermination trenches 2703-1 and 2703-2 having a relatively larger radiusof curvature. The spacing between the trenches can also be adjustedbased on the device voltage requirements. In this embodiment, thedistance S1 between termination trenches 2703-1 and 2703-2 isapproximately twice the distance between the first termination trench2703-1 and the end of the active trenches.

FIGS. 28A, 28B, 28C, and 28D show exemplary cross-sectional views forvarious termination regions with silicon pillar charge balancestructures. In the embodiment shown in FIG. 28A, field plates 2809Acontact every ring of p-type pillar 2803A. This allows wider mesaregions because of lateral depletion due to field plates. The breakdownvoltage generally depends on the field oxide thickness, the number ofrings and the depth and spacing of termination pillars 2803A. Manydifferent variations for this type of termination structure arepossible. For example, FIG. 28B shows an alternative embodiment whereina large field plate 2809B-1 covers all the pillars 2803B except the lastpillar, which is connected to another field plate 2809B-2. By groundinglarge field plate 2809B-1, the mesa regions between the p-type pillarsdeplete quickly and the horizontal voltage drop will not be significant,causing lower breakdown voltage than the embodiment shown in FIG. 28A.In another embodiment shown in FIG. 28C, the termination structure hasno field plates on the middle pillars. Because there is no field plateon the middle pillars, they have narrower mesa region to depleteadequately. In one embodiment, a gradually decreasing mesa width towardsthe outer ring yields optimal performance. The embodiment shown in FIG.28D facilitates contact to p-type pillars by providing a wider wellregion 2808D and increasing the spacing between the field oxide layersas shown.

In the case of ultra-high voltage devices that employ varioussuper-junction techniques of the type described above, the breakdownvoltage is much higher than the conventional BVpp. For a super-junctiondevice, the charge balance or super-junction structures (e.g., oppositepolarity pillars or floating regions, buried electrodes, etc.) are alsoused in the termination region. Standard edge termination structures incombination with charge balance structures, such as field plates on thetop surface at the edge of the device can also be used. In someembodiments, standard edge structures on the top can be eliminated byusing a rapidly decreasing charge in the termination junction. Forexample, p-type pillars in the termination region can be formed withdecreasing charge the farther they get from the active area creating anet n-type balance charge.

In one embodiment, the spacing between the p-type pillars in thetermination region is varied as the pillars move farther away from theactive regions. A highly simplified cross-sectional view of oneexemplary embodiment of a device 2900A according to this embodiment isshown in FIG. 29A. In the active area of device 2900A, oppositeconductivity pillars 2926A made of, for example, multiple connectedp-type spheres are formed under the p-type well 2908A in n-type driftregion 2904A. At the edge of the device, under the termination region,p-type termination pillars TP1, TP2, to TPn are formed as shown. Insteadof having uniform spacing as in the active area, the center-to-centerspacing between termination pillars TP1 to TPn increases as the pillarsmove farther away from the interface with the active region. That is,distance D1 between TP2 and TP3 is smaller than distance D2 between TP3and TP4, and distance D2 is smaller than distance D3 between TP4 andTP5, and so on.

Several variations of this type of super-junction termination structureare possible. For example, instead of forming p-type termination pillarsTP1-TPn at varying distances inside voltage sustaining layer 2904A, thecenter-to-center spacing could remain uniform but the width of eachtermination pillar could vary. FIG. 29B shows a simplified example of atermination structure according to this embodiment. In this example,termination pillar TP1 has a width W1 that is larger than the width W2of termination pillar TP2, and W2 in turn is made larger than the widthW3 of termination pillar TP3 and so on. In terms of the spacing betweenthe opposite polarity charge balancing regions in the terminationregion, the resulting structure in device 2900B is similar to that ofdevice 2900A, even though in device 2900B the center-to-center spacingbetween trench pillars may be the same. In another exemplary embodimentshown in a simplified cross-sectional view in FIG. 29C, the width ofeach opposite polarity pillar 2926C in the active region is decreasedfrom the top surface to the substrate, whereas the width for terminationpillars TP1 and TP2 remains substantially the same. This achieves thedesired breakdown voltage while utilizing less area. Those skilled inthe art appreciate that the various termination structures describedabove can be combined in any desired manner, including for example, thecenter-to-center spacing and/or the overall width of termination pillarsin device 2900C shown in FIG. 29C can be varied as described inconnection with the embodiments shown in FIGS. 29A and 29B.

Process Techniques

A number of different devices with trench structures having multipleburied electrodes or diodes have thus far been described. In order tobias these trenched electrodes, these devices allow for electricalcontact to be made to each of the buried layers. A number of methods forforming the trench structures with buried electrodes and for makingcontact to the buried poly layers inside the trenches are disclosedherein. In one embodiment, contacts to trenched poly layers are made atthe edge of the die. FIG. 30A shows one example of edge contacting for atrench device 3000 with two poly layers 3010 and 3020. FIG. 30A depictsa cross-sectional view of the device along the longitudinal axis of atrench. According to this embodiment, where the trench terminates nearthe edge of the die, poly layers 3010 and 3020 are brought up to thesurface of the substrate for contact purposes. Openings 3012 and 3022 indielectric (or oxide) layers 3030 and 3040 allow for metal contact tothe poly layers. FIGS. 30B to 30F illustrate various processing stepsinvolved in forming the edge contact structure of FIG. 30A. In FIG. 30B,a dielectric (e.g., silicon dioxide) layer 3001 is patterned on top ofepitaxial layer 3006, and the exposed surface of the substrate is etchedto form trench 3002. A first oxide layer 3003 is then formed across thetop surface of the substrate including the trench as shown in FIG. 30C.A first layer of conductive material (e.g., polysilicon) 3010 is thenformed on top of oxide layer 3003 as shown in FIG. 30D. Referring toFIG. 30E, poly layer 3010 is etched away inside the trench and anotheroxide layer 3030 is formed over poly 3010. Similar steps are carried outto form the second oxide-poly-oxide sandwich as shown in FIG. 30F, wherethe top oxide layer 3040 is shown as being etched to make openings 3012and 3022 for metal contact layer to poly layers 3010 and 3020,respectively. The last steps can be repeated for additional poly layers,and poly layers can be tied together by the overlying metal layer ifdesired.

In another embodiment, contacts to multiple poly layers in a giventrench are made in the active area of the device instead of along theedge of the die. FIG. 31A depicts one example of the active area contactstructure for multiple buried poly layers. In this example, across-sectional view along the longitudinal axis of the trench shows apoly layer 3110 which provides the gate terminal and poly layers 3111 aand 3111 b that provide two shield layers. While three separate metallines 3112, 3122 and 3132 are shown as making contact to the shield polylayers, they can be all tied together and connected to the sourceterminal of the device, or any other contacting combination can be usedas required by a particular application. An advantage of this structureis the planar nature of the contact as compared to the multi-layer edgecontact structure shown in FIG. 30A.

FIGS. 31B to 31M illustrate one example of a process flow for forming anactive area shield contact structure for a trench with two layers ofpoly. Etching of trenches 3102 in FIG. 31B is followed by shield oxide3108 formation in FIG. 31C. Shield polysilicon 3111 is then depositedand recessed inside the trenches as shown in FIG. 31D. Shield poly 3111is additionally recessed in FIG. 31E, except for locations where shieldcontact at the surface of the substrate is desired. In FIG. 31E, a mask3109 protects the poly inside the middle trench from further etch. Inone embodiment, this mask is applied at different locations alongdifferent trenches such that for the middle trench, for example, shieldpoly is recessed in other portions of the trench in the third dimension(not shown). In another embodiment, shield poly 3111 inside one or moreselect trenches in the active area is masked along the entire length ofthe trench. Shield oxide 3108 is then etched as shown in FIG. 31F, and athin layer of gate oxide 3108 a is then formed across the top of thesubstrate after mask 3109 is removed as shown in FIG. 31G. This isfollowed by gate poly deposition and recess (FIG. 31H), p well implantand drive (FIG. 31I), and n+ source implant (FIG. 31J). FIGS. 31K, 31Land 31M depict the steps of BPSG deposition, contact etch and p+ heavybody implant, followed by metallization, respectively. FIG. 31N shows across-sectional view of an alternative embodiment for an active areashield contact structure wherein shield poly 3111 forms a relativelywider platform on top of shield oxide. This facilitates contacting theshield poly, but introduces topography that may further complicate thefabrication process.

A simplified top-down layout view of an exemplary trench device with anactive area shield contact structure is shown in FIG. 32A. A maskdefining shield poly recess prevents the recessing of the shield poly atlocation 3211C in the active region as well as in perimeter shieldtrench 3213. A modification of this technique uses a “dogbone”-likeshape for the shield poly recess mask that provides a wider region atthe intersection with each trench 3202 for contact to the shield poly.This allows the shield poly in the masked region to also be recessed butto the original surface of the mesa, thus eliminating topography. Thetop-down layout view for an alternative embodiment is shown in FIG. 32B,wherein active area trenches are connected to the perimeter trench. Inthis embodiment, the shield poly recess mask prevents recessing ofshield poly along the length of a selected trench (middle trench in theexample shown) for active area shield trench contact to source metal.FIGS. 32C and 32D are simplified layout diagrams showing two differentembodiments for making contact to the perimeter trench in a trenchdevice with broken trench structure. In these figures, active trenches3202 and perimeter trench 3213 are depicted by a single line forillustrative purposes. In FIG. 32C, extensions or fingers from perimetergate poly runner 3210 are staggered with respect to perimeter shieldpoly fingers to space the perimeter contacts away from the perimetertrench. Source and shield contact area 3215 also makes contact to shieldpoly in the active region in locations 3211C as shown. The embodimentshown in FIG. 32D eliminates the offset between active and perimetertrenches to avoid possible limitations arising from trench pitchrequirements. In this embodiment, active trenches 3202 and horizontalextensions from perimeter trench 3213 are aligned, and windows 3217 ingate poly runner 3210 allow for contacts to be made to shield polyaround the perimeter. Active area contacts are made in locations 3211Cas in previous embodiments.

An alternative embodiment for contacting trenched shield poly layers inthe active area is shown in FIG. 33A. In this embodiment, instead ofrecessing the shield poly it extends vertically over a substantial partof the active trench up to the silicon surface. Referring to FIG. 33A,shield poly 3311 splits gate poly 3310 into two as it extends verticallyalong the height of trench 3302. The two gate poly segments areconnected in the third dimension at a suitable location inside thetrench or as they exit the trench. One advantage of this embodiment isthe area that is saved by making source poly contact inside the activetrench instead of using silicon space that would be dedicated for thetrenched poly contact. FIGS. 33B to 33M illustrate one example of aprocess flow for forming an active area shield contact structure of thetype shown in FIG. 33A. Etching of trenches 3302 in FIG. 33B is followedby shield oxide 3308 formation in FIG. 33C. Shield polysilicon 3311 isthen deposited inside the trenches as shown in FIG. 33D. Shield poly3311 is etched and recessed inside the trenches as shown in FIG. 33E.Shield oxide 3308 is then etched as shown in FIG. 33F, leaving anexposed portion of shield polysilicon 3311 that forms two troughs on itssides inside the trench. A thin layer of gate oxide 3308 a is thenformed across the top of the substrate, sidewalls of the trenches andtroughs inside the trenches as shown in FIG. 33G. This is followed bygate poly deposition and recess (FIG. 33H), p-well implant and drive(FIG. 33I), and n+ source implant (FIG. 33J). FIGS. 33K, 33L and 33Mdepict the steps of BPSG deposition, contact etch and p+ heavy bodyimplant, followed by metallization, respectively. Variations on thisprocess flow are possible. For example, by re-ordering some of theprocess steps, the process steps forming gate poly 3310 can be conductedprior to the steps forming shield poly 3311.

Specific process recipes and parameters and variations thereof forperforming many of the steps in the above process flows are well-known.For a given application, certain process recipes, chemistries andmaterial types can be fine tuned to enhance manufacturability andperformance of the device. Improvements can be made from the startingmaterial, i.e., the substrate on top of which the epitaxial (epi) driftregion is formed. In most power applications, reduction in thetransistor on-resistance R_(DSon) is desirable. The ideal on-resistanceof a power transistor is a strong function of the critical field whichis defined as the maximum electric field in the device under breakdownconditions. Transistor specific on-resistance can be significantlyreduced if the device is fabricated in a material with critical fieldhigher than that of silicon, provided that reasonable mobility ismaintained. While many of the power devices features, includingstructures and processes, described thus far have been described in thecontext of a silicon substrate, other embodiments using substratematerial other than silicon are possible. According to one embodiment,the power devices described herein are fabricated in a substrate made ofwide-bandgap material, including for example, silicon carbide (SiC),gallium nitride (GaN), gallium arsenide (GaAs), indium phosphide (InP),diamond and the like. These wide-bandgap materials exhibit a criticalfield that is higher than the critical field for silicon and can allowfor a significant reduction in transistor on-resistance.

Another primary contributor to the transistor on-resistance is thethickness and doping concentration of the drift region. The drift regionis typically formed by epitaxially grown silicon. To reduce R_(DSon), itis desirable to minimize the thickness of this epi drift region. Thethickness of the epi layer is dictated in part by the type of startingsubstrate. For example, a red-phosphorus doped substrate is a commontype of starting substrate material for discrete semiconductor devices.A property of phosphorus atoms, however, is that they diffuse quickly insilicon. The thickness of the epi region that is formed on top of thesubstrate is therefore determined to accommodate the up-diffusion ofphosphorus atoms from the underlying heavily doped substrate.

In order to minimize the thickness of the epi layer, according to oneembodiment shown in FIG. 34, an epi spacer or buffer (or barrier) layer3415 having dopants with relatively less diffusivity such as Arsenic, isformed above a phosphorus substrate 3414. The combined phosphorous-dopedsubstrate and Arsenic-doped buffer layer provides the foundation for thesubsequent formation of epi drift region 3406. The Arsenic dopantconcentration in layer 3415 is determined by the breakdown voltagerequirements of the device, and the thickness of Arsenic epi layer 3415is determined by specific thermal budget. A regular epi layer 3406 maythen be deposited on top of the Arsenic epi, the thickness of whichwould be determined by device requirements. The much lower diffusivityof Arsenic allows the overall thickness of the epi drift region bereduced resulting in a reduction in transistor on-resistance.

In an alternative embodiment, in order to counter the up-diffusion ofdopant species from the heavily doped substrate to the epi layer adiffusion barrier is employed between the two layers. According to oneexemplary embodiment shown in FIG. 35, a barrier layer 3515 composed of,e.g., silicon carbide Si_(x)C_(1-x) is deposited epitaxially on eitherboron or phosphorus substrates 3514. Epi layer 3506 is then depositedatop barrier layer 3515. The thickness and carbon composition may varyaccording to the thermal budget of the process technology. Alternately,carbon dopants can be first implanted into substrate 3514, after whichthermal treatment activates the carbon atoms to form a Si_(x)C_(1-x)compound at the surface of substrate 3514.

Another aspect of certain trench transistor technologies that limits theability to reduce the thickness of the epi is the junction formedbetween the deep body and epi layer that is sometimes employed in theactive region and sometimes in the termination region. The formation ofthis deep body region commonly involves an implant step early in theprocess. Due to the large subsequent thermal budget required by theformation of field oxide and gate oxide, the junction at the deep bodyand drift region is graded to a large extent. To avoid early breakdownat the edge of the die, a much thicker drift region is needed whichresults in higher on-resistance. The use of a diffusion barrier layercan also be employed at the deep body-epi junction in order to minimizethe required epi thickness. According to an exemplary embodiment shownin FIG. 36, carbon dopants are implanted through the deep body windowand before the deep body implant is carried out. The subsequent thermalprocess activates the carbon atoms to form a layer of Si_(x)C_(1-x)compound 3615 at the boundary of deep body region 3630. Silicon carbidelayer 3615 serves as a diffusion barrier preventing boron diffusion. Theresulting deep body junction is shallower allowing the thickness of epilayer 3606 to be reduced. Yet another junction in a typical trenchtransistor that can benefit from a diffusion barrier is the well-driftregion junction. A simplified example of an embodiment employing such abarrier layer is shown in FIG. 37. In the exemplary process flow for thestructure of FIG. 31M, a p-well is formed between the two steps shown inFIGS. 31H and 31I. Prior to implanting the well dopants (p-type for thisexemplary n-channel embodiment), carbon is implanted first. Thesubsequent thermal process activates the carbon atoms to form a layer3715 of Si_(x)C_(1-x) at the p-well epi junction. Layer 3715 serves as adiffusion barrier to prevent boron diffusion so that the depth of p-well3704 can be preserved. This helps reduce transistor channel lengthwithout increasing the potential for reach-through. Reach-through occurswhen the edge of the advancing depletion boundary reaches the sourcejunction as the drain-source voltage increases. By acting as a diffusionbarrier, layer 3715 also prevents reach-through.

As discussed above, reducing the transistor channel length is desirablebecause it results in reduced on-resistance. In another embodiment,transistor channel length is minimized by forming the well region usingepitaxially grown silicon. That is, instead of the conventional methodof forming the well that involves an implant into the drift epi layerfollowed by a diffusion step, the well region is formed on top of theepi drift layer. There are advantages other than a shorter channellength that can be obtained from an epi-well formation. In shielded gatetrench transistors, for example, the distance by which the gateelectrode extends below the bottom of the well where it meets the trench(gate to drain overlap) is critical in determining gate charge Qgd. Gatecharge Qgd directly impacts the switching speed of the transistor. It isdesirable, therefore, to be able to accurately minimize and control thisdistance. However, in fabrication processes where the well is implantedand diffused into the epi as shown, for example, in FIG. 31I above, thisdistance is difficult to control.

To better control the gate-to-drain overlap at the corner of the well,various methods for forming a trench device with a self-aligned well areproposed. In one embodiment, a process flow involving deposition of anepi-well enables the self-alignment of the bottom of the body junctionto the bottom of the gate. Referring to FIGS. 38A-38D, there is shown asimplified process flow for one example of a self-aligned epi-welltrench device with buried electrode (or shielded gate). A trench 3802 isetched into a first epi layer 3806 that is formed on top of substrate3814. For an n-channel transistor, substrate 3814 and first epi layer3806 are of n-type material.

FIG. 38A shows a layer of shield dielectric 3808S grown on the topsurface of epi layer 3806 including inside trench 3802. Conductivematerial 3811 such as polysilicon is then deposited inside trench 3802and etched back below the epi mesa as shown in FIG. 38B. Additionaldielectric material 3809S is deposited to cover shield poly 3811. Afteretching back the dielectric to clear the mesa, a second layer of epi3804 is selectively grown on top of first epi layer 3806, as shown inFIG. 38C. The mesas formed by epi layer 3804 create an upper trenchportion above original trench 3802 as shown. This second epi layer 3804has dopants of opposite polarity (e.g., p-type) to that of the first epilayer 3806. The dopant concentration in second epi layer 3804 is set tothe desired level for the transistor well region. After the step ofselective epi growth (SEG) that forms layer 3804, a layer of gatedielectric 3808G is formed on the top surface and along the trenchsidewalls. Gate conductive material (poly) is then deposited to fill theremaining portion of trench 3802, and then planarized as shown in FIG.38D. The process continues as in, for example, the process flow shown inFIGS. 31J to 31M to complete the transistor structure.

As shown in FIG. 38D, this process results in gate poly 3810 that isself-aligned with well epi 3804. To lower the bottom of gate poly 3810below epi well 3804, the top surface of inter-poly dielectric layer3809S as shown in FIG. 38C can be etched slightly to the desiredlocation inside trench 3802. This process, therefore, provides foraccurate control of the distance between the bottom of the gateelectrode and the well corner. Those skilled in the art appreciate thatthe SEG well formation process is not limited to a shielded gate trenchtransistor and can be employed in many other trench gate transistorstructures several of which have been described herein. Other methods offorming SEG mesa structures are described in commonly-assigned U.S. Pat.Nos. 6,391,699 to Madson et al. and 6,373,098, to Brush et al., whichare hereby incorporated by reference in their entirety.

An alternative method for controlling the corner of the well forself-alignment purposes does not rely on SEG well formation and insteademploys a process involving angled well implant. FIGS. 39A and 39Billustrate an exemplary process flow for this embodiment. Instead offorming the well after the trench is filled with gate poly as shown, forexample, in FIGS. 31H and 31I, in this embodiment a first well implant3905 at a given partial dose is performed after embedding shield poly indielectric layer 3908 inside trench 3902 and before the remainingportion of the trench is filled. A second but angled well implant isthen performed through the sidewalls of trench 3902 as shown in FIG.39B. The drive cycle is then completed to obtain the desired contour forthe well to drift epi interface at the trench corner. The implant dose,energy and the particulars of the drive cycles will vary depending onthe structural requirements of the device. This technique can beemployed in a number of different device types. In an alternativeembodiment, the trench pitch and the angle implant are adjusted suchthat when the angle implant is diffused, it merges with the region froma neighboring cell to form a continuous well, eliminating the need forthe first well implant.

Another embodiment for a self-aligned epi well process for forming atrench device is described in connection with FIGS. 40A to 40E. Asdiscussed above, to reduce gate-to-drain capacitance some trench gatetransistors employ a gate dielectric layer that is thicker at the bottomof the trench below the gate poly than the dielectric layer along theinner vertical sidewalls. According to the exemplary process embodimentshown in FIGS. 40A to 40E, a dielectric layer 4008B is first formed ontop of an epi drift layer 4006 as shown in FIG. 40A. Dielectric layer4208B is formed with the desired thickness for the bottom of the trench,and is then etched leaving dielectric columns, as shown in FIG. 40B,that have the same width as the trench that will subsequently be formed.Next, in FIG. 40C, a selective epi growth step is performed to form asecond epi drift layer 4006-1 around dielectric columns 4008B. Seconddrift epi layer 4006-1 is of the same conductivity type and can be ofthe same material as the first epi drift layer 4006. Alternatively, itis possible to use other types of materials for second epi drift layer4006-1. In one exemplary embodiment, second drift epi layer 4006-1 isformed by an SEG step strained with a silicon germanium (Si_(x)Ge_(1-x))alloy. The SiGe alloy improves the carrier mobility at the accumulationregion near the bottom of the trench. This improves the switching speedof the transistor and reduces R_(DSon). The use of other compounds suchas GaAs or GaN are also possible.

A blanket epi well layer 4004 is then formed on the top surface, and isthen etched to form trenches 4002 as shown in FIGS. 40D and 40E,respectively. This is followed by gate oxide formation and gate polydeposition (not shown). The resulting structure is a trench gate with aself-aligned epi well. Conventional processing techniques can be used tocomplete the remaining process steps. Those skilled in the artappreciate that variations are possible. For example, instead of forminga blanket epi well layer 4004 and then etching trenches 4002, epi well4002 can be selectively grown only on top of second drift epi layer4006-1 forming trenches 4002 as it grows.

The various processing techniques described above enhance deviceperformance by focusing on the formation of the well region to reducechannel length and R_(DSon). Similar enhancements can be achieved byimproving other aspects of the process flow. For example, deviceresistance can be further reduced by reducing the substrate thickness. Awafer thinning process is therefore commonly performed in order toreduce the thickness of the substrate. Wafer thinning is typicallyperformed by mechanical grind and tape processes. The grind and tapeprocesses impose mechanical forces on the wafer that cause damage to thewafer surface resulting in manufacturing problems.

In an embodiment described hereinafter, an improved wafer thinningprocess significantly reduces substrate resistance. Illustrated in FIGS.40R, 40S, 40T and 40U, is one method for reducing the thickness of thesubstrate. After finishing the fabrication of the desired circuitry on awafer, the top of the wafer where the circuitry is fabricated istemporarily bonded to a carrier. FIG. 40R shows a finished wafer 4001that is bonded to a carrier 4005 by a bonding material 4003. Thebackside of the finished wafer is then polished to the desired thicknessusing a process such as grinding, chemical etching or the like. FIG. 40Sshows the same sandwich as FIG. 40R with finished wafer 4001 having beenthinned. After the polishing of the backside of wafer 4001, the backsideof the wafer is bonded to a low resistance (e.g., metal) wafer 4009 asshown in FIG. 40T. This could be accomplished using conventional methodsusing, for example, a thin coating of solder 4007 to bond metal wafer4009 to thinned finished wafer 4001 under temperature and pressure.Carrier 4005 is then removed and the top surface of thinned finishedwafer 4001 is cleaned before further processing. The highly conductivemetal substrate 4009 facilitates heat dissipation, reduction inresistance and provides mechanical strength for the thinned wafer.

An alternative embodiment achieves thinner wafers without the drawbacksof the conventional mechanical processes by performing the finalthinning step using a chemical process. According to this embodiment,active devices are formed in silicon layers of a silicon-on-thick-glass(SOTG) substrate. At the grinding stage, the wafer can be thinned bychemically etching away glass at the backside of SOTG substrate. FIG. 41sets out an exemplary process flow according to this embodiment.Starting from a silicon substrate, first at step 4110 a dopant such as,e.g., He or H₂ is implanted into the silicon substrate. Next, at 4112,the silicon substrate is bonded to a glass substrate. Different bondingprocesses can be used. In one example, a silicon wafer and a glass waferare sandwiched and heated up to around, e.g., 400 C to bond the twosubstrates. The glass can be, e.g., silicon dioxide and the like, andmay have a thickness of, for example, about 600 um. This is followed byan optional cleaving of the silicon substrate at 4114 and forming of theSOTG substrate. To protect the substrate from stress during handling andsubsequent processing, the bonding process can be repeated to form SOTGlayer on the other side of the substrate (step 4116). An epi layer isnext deposited on the silicon surface of the substrate (step 4118). Thiscan be performed on the backside in addition to the front side. Thedoping level of the backside epi is preferably similar to that of thebackside silicon, while the front-side epi is doped as required by thedevice. The substrate is then subjected to the various steps in thefabrication process for forming the active device on the front-sidesilicon layers.

In one embodiment, to further enhance the substrate strength inwithstanding stress introduced by front-side processing steps, thebackside substrate can be patterned to approximate an inverse structureof the front-side die frame. In this way, the glass substrate is etchedinto a grid to help the thin substrate sustain the stress in the wafer.At grinding, first the silicon layer from the back side is removed by aconventional grinding process (step 4120). This is followed by anothergrinding step 4122 that removes a portion (e.g., half) of the glass. Theremaining portion of the glass is then removed by a chemical etchprocess using, e.g., hydrofluoric acid. The etching of the backsideglass can be performed without the risk of attacking or causingmechanical damage to active silicon layers. This eliminates the need fortaping the wafer, which eliminates the need for tape and re-tapeequipment and the process risks associate with each of those operations.Accordingly, this process allows for further minimizing substratethickness to enhance device performance. It is to be understood thatmany variations of this improved wafer thinning process are possible.For example, depending on the desired thickness for the final substrate,the thinning step may or may not involve grinding and chemical etch maybe sufficient. Also, the improved wafer thinning process is not limitedto processing of discrete devices and can be utilized in the processingof other types of devices. Other wafer thinning processes are describedin commonly-assigned U.S. Pat. No. 6,500,764, to Pritchett, which ishereby incorporated in its entirety.

There are a number of other structural and processing aspects of thepower transistor and other power devices that can significantly impacttheir performance. The shape of the trench is one example. To reduce thepotentially damaging electric fields that tend to concentrate around thecorners of the trench, it is desirable to avoid sharp corners andinstead form trenches that have rounded corners. To improve reliability,it is also desirable to have trench sidewalls with smooth surfaces. Thedifferent etch chemistries offer trade-offs among several responses,such as: silicon etch rate, selectivity to the mask layer, etch profile(sidewall angle), top corner rounding, sidewall roughness, and roundingof the trench bottom. A fluorinated chemistry, for example SF6, providesa high silicon etch rate (greater than 1.5 um/min), rounded trenchbottoms, and a straight profile. The drawbacks of the fluorinatedchemistry are rough sidewalls and difficulties with control of the topof the trench (can be re-entrant). A chlorinated chemistry, for exampleCl₂, provides smoother sidewalls, and better control of the etch profileand the top of the trench. The trade-offs with the chlorinated chemistryare lower silicon etch rate (less than 1.0 um/min), and less rounding ofthe trench bottom.

Additional gases can be added to each chemistry to aid in passivatingthe sidewall during the etch. Sidewall passivation is used to minimizelateral etching, while etching to the desired trench depth. Additionalprocessing steps can be used to smooth the trench sidewalls, and achieverounding of the top corner and the bottom of the trench. The surfacequality of trench sidewalls is important because it affects the qualityof an oxide layer that can be grown on the trench sidewall. Regardlessof the chemistry used, a breakthrough step is typically used before themain etch step. The purpose of the breakthrough step is to remove anynative oxide on the surface of the silicon that may mask the etching ofthe silicon during the main etch step. Typical breakthrough etchchemistries involve CF₄ or Cl₂.

One embodiment for an improved etch process shown in FIG. 42A employs achlorine based main silicon trench etch followed by a fluorine basedetch step. One example of this process employs Cl₂/HBr main etch stepfollowed by an SF₆ etch step. The chlorinated step is used to etch themain trench to a portion of the desired depth. This defines the trenchprofile with some degree of taper, and with smooth sidewalls. Thesubsequent fluorinated step is used to etch the remainder of the trenchdepth, rounding the trench bottom, and providing further smoothing ofany dangling silicon bonds on the trench sidewall. The fluorinated etchstep is preferably performed at relatively low fluorine flow, lowpressure, and low power to control the smoothing and the rounding. Dueto the difference in etch rates between the two etch chemistries, thetimes of the two steps can be balanced to achieve a more reliable andmanufacturable process with an acceptable overall etch time, whilemaintaining the desired trench profile, sidewall roughness, and trenchbottom rounding.

In another embodiment shown in FIG. 42B, an improved method for siliconetch includes a fluorine based main etch step followed by a chlorinebased second etch step. One example of this process uses an SF₆/O₂ mainetch followed by a Cl₂ step. The fluorine step is used to etch the maintrench for the majority of the depth. This step produces a trench with astraight sidewall and with a rounded trench bottom. Optionally, oxygencould be added to this step to provide sidewall passivation, and to aidin maintaining a straight sidewall by reducing lateral etching. Achlorine follow-up step rounds the top corners of the trench and reducesthe roughness of the sidewall. The high silicon etch rate of thefluorine step increases the manufacturability of the process byincreasing the throughput of the etch system.

In yet another embodiment, shown in FIG. 42C, an improved silicon etchprocess is obtained by adding argon to a fluorine based chemistry. Anexample of a chemistry used for the main etch step according to thisembodiment is SF₆/O₂/Ar. The addition of argon to the etch stepincreases ion bombardment and therefore makes the etch more physical.This helps with controlling the top of the trench, and eliminates thetendency for the top of the trench to be re-entrant. The addition ofargon may also increase the rounding of the trench bottom. An additionaletch process may be needed for sidewall smoothing.

An alternative embodiment for an improved silicon etch process uses afluorine based chemistry with oxygen removed from the start of the mainetch step, as shown in FIG. 42D. One example of this process uses an SF₆step followed by an SF₆/O₂ step. In the first phase of the etch there isa lack of sidewall passivation due to the absence of O₂. This results inan increase in the amount of lateral etching at the top of the trench.Then the second etch step, SF₆/O₂, continues etching the remainder ofthe trench depth with a straight profile, and a rounded trench bottom.This results in a trench structure that is wider at the top, sometimesreferred to as a T-trench. Examples of devices utilizing a T-trenchstructure are described in detail in commonly-assigned U.S. patentapplication Ser. No. 10/442,670, entitled “Structure and Method forForming a Trench MOSFET Having Self-Aligned Features,” by RobertHerrick, hereby incorporated by reference in its entirety. The timeperiods for the two main etch steps can be adjusted to achieve thedesired depth for each portion of the T-Trench (top T portion, bottomstraight sidewall portion). Additional processing could be used to roundoff the top corner of the T-Trench and smooth the trench sidewalls.These additional processing methods could include, for example: (1) afluorine-based step at the end of the trench etch recipe, or (2) aseparate fluorine-based etch on a separate etch system, or (3) asacrificial oxide, or any other combination. A chemical mechanicalplanarization (CMP) step could be used to remove the top re-entrantportion of the trench profile. An H₂ anneal can also be used to assistin rounding and making favorable slope trench profile.

For high voltage applications where trenches tend to be deeper, thereare additional considerations. For example, due to the deeper trenchesthe silicon etch rate is important to produce a manufacturable process.The etch chemistry for this application is typically a fluorinatedchemistry, because a chlorinated etch chemistry is too slow. Also astraight-to-tapered trench profile is desired, with smooth sidewalls.Due to the depth of the trench, the etch process needs to also haveexcellent selectivity to the mask layer. If the selectivity is poor thena thicker mask layer is required, which increases the overall aspectratio of the feature. Sidewall passivation is also very critical; a finebalance needs to be achieved. Too much sidewall passivation will causethe bottom of the trench to narrow to the point where it closes; toolittle sidewall passivation will result in the increased lateraletching.

In one embodiment, a deep trench etch process is provided that optimallybalances all of these requirements. According to this embodiment, shownin FIG. 42E, the etch process includes a fluorine-based chemistry withramped O₂, ramped power, and/or ramped pressure. One exemplaryembodiment uses a SF₆/O₂ etch step in a manner that maintains etchprofile and silicon etch rate throughout the etch. By ramping the O₂,the amount of sidewall passivation can be controlled throughout the etchto avoid increased lateral etching (in case of too little passivation)or pinching off of the bottom of the trench (in case of too muchpassivation). Examples of using fluorine based etch with ramping oxygengas flows are described in detail in commonly owned U.S. Pat. No.6,680,232, entitled “Integrated Circuit Trench Etch with IncrementalOxygen Flow,” by Grebs et al., which is hereby incorporated byreference. Ramping of the power and the pressure will aid in controllingthe ion flux density and in maintaining the silicon etch rate. If thesilicon etch rate decreases significantly during the etch as the trenchis etched deeper, the total etch time will increase. This will result ina low wafer throughput for the process on the etcher. Also, ramping theO₂ may help in controlling the selectivity to the masking material. Anexemplary process according to this embodiment for trenches that aredeeper than, e.g., 10 μm, may have an O₂ flow rate of 3 to 5 sccm perminute, at a power level of 10-20 watts per minute and pressure level of2-3 mT per minute.

An alternative embodiment of a deep trench etch process uses a moreaggressive fluorine based chemistry such as NF₃. Since NF₃ is morereactive than SF₆ for silicon etching, an increased silicon etch ratecould be achieved with an NF3 process. Additional gases may need to beadded for sidewall passivation and profile control.

In another embodiment, an NF₃ etch step is followed by an SF₆/O₂process. According to this embodiment, the NF₃ step is used to etch themajority of the trench depth with a high silicon etch rate. Then theSF₆/O₂ etch step is used to passivate the existing trench sidewall, andetch the remaining portion of the trench depth. In a variation of thisembodiment shown in FIG. 42F, NF₃ and SF₆/O₂ etch steps are performed inan alternating fashion. This yields a process with a higher silicon etchrate than a straight SF₆/O₂ process. It balances between a fast etchrate step (NF₃), and a step that generates sidewall passivation (SF₆/O₂)for profile control. The balance of the steps controls the sidewallroughness. There may also be a need to ramp the O₂, power, and pressurefor the SF₆/O₂ portion of the etch to maintain the silicon etch rate,and to generate enough sidewall passivation to aid in controlling theetch profile. Those skilled in the art will appreciate that the variousprocess steps described in connection with the above embodiments can becombined in different ways to achieve optimum trench etch processing. Itis to be understood that these trench etch processes can be employed forany of the trenches in any of the power devices described herein, aswell as for any other type of trench used in other types of integratedcircuits.

Prior to the trench etch process, a trench etch mask is formed on thesurface of the silicon and is patterned to expose areas to be trenched.As shown in FIG. 43A, in a typical device the trench etch first etchesthrough a layer of nitride 4305 and another thin layer of pad oxide 4303before it etches the silicon substrate. After the trench is formedduring the formation of an oxide layer in the trench, pad oxide 4303 canalso grow at the edge of the trench lifting the overlying nitride layer.This results in what is commonly referred to as a “bird's beak”structure 4307 as the pad oxide grows locally near the trench edge undernitride layer 4305. The source region that will be subsequently formednext to the trench edge under the pad oxide with the bird's beakstructure will be shallower near the trench. This is highly undesirable.To eliminate the bird's beak effect, in one embodiment, shown in FIG.43B, a layer of non-oxidizing material such as polysilicon 4309 issandwiched between nitride layer 4305 and pad oxide 4303. Poly layer4309 prevents pad oxide 4303 from further oxidization during subsequenttrench oxide formation. In another embodiment, shown in FIG. 44A, afteretching through nitride layer 4405 and pad oxide 4403 defining thetrench opening, a thin layer of non-oxidizing material 4405-1 such asnitride is formed on the surface structure. Protective layer 4405-1 isthen removed from the horizontal surfaces leaving spacers along thevertical edges of the nitride-pad oxide structure as shown in FIG. 44B.The nitride spacers protect pad oxide 4403 from further oxidation duringsubsequent steps reducing the bird's beak effect. In an alternativeembodiment, to reduce the degree of any bird's beak formation bothembodiments shown in FIGS. 43B and 44B can be combined. That is, a layerof polysilicon can be sandwiched between pad oxide and the overlyingnitride in addition to the spacers that result from the processdescribed in connection with FIGS. 44A and 44B. Other variations arepossible, including, for example, adding another layer (e.g., oxide) ontop of the nitride to aid in the nitride selectivity while etchingsilicon trenches.

As described above in connection with various transistors with shieldedgate structures, a layer of dielectric material isolates the shieldelectrode from the gate electrode. This inter-electrode dielectric layerthat is sometimes referred to as the inter-poly dielectric or IPD mustbe formed in a robust and reliable fashion so that it can withstand thepotential difference that may exist between the shield electrode and thegate electrode. Referring back to FIGS. 31E, 31F and 31G, there is showna simplified flow for the relevant process steps. After the etch back ofshield poly 3111 inside the trench (FIG. 31E), shield dielectric layer3108 is etched back to the same level as shield poly 3111 (FIG. 31F).Gate dielectric layer 3108 a is then formed on the top surface of thesilicon as shown in FIG. 31G. It is this step that forms the IPD layer.An artifact of the shield dielectric recess etch is the formation ofshallow troughs on the top surface of the shield dielectric remaining oneither sides of the shield electrode. This is shown in FIG. 45A. Theresulting structure with the uneven topography can cause conformalityproblems, especially with subsequent filling steps. In order toeliminate such problems, various improved methods for forming the IPDare presented.

According to one embodiment, after the shield dielectric recess etch, apolycrystalline silicon (poly) liner 4508P is deposited as shown in FIG.45B using, for example, a low pressure chemical vapor deposition (LPCVD)process. Alternatively, poly liner 4508P can be formed only over theshield poly and shield dielectric and leaving trench sidewallssubstantially free of poly by using a selective growth process for polyor collimated sputtering of poly. Poly liner 4508P is subsequentlyoxidized converting it into silicon dioxide. This can be performed by aconventional thermal oxidation process. In the embodiment where no polyis formed on the trench sidewalls, this oxidation process also formsgate dielectric layer 4508G. Otherwise, after etching the oxidized polylayer from the sidewalls of the trench, a thin layer of gate dielectric4508G is formed and the remaining trench cavity is filled with gateelectrode 4510 as shown in FIG. 45C. An advantage of this process isthat poly deposits in a very conformal fashion. This minimizes voids andother defects and creates a more even surface once poly is deposited ontop of the shield dielectric and shield electrode. The result is animproved IPD layer that is more robust and reliable. By lining thetrench sidewalls and the adjacent silicon surface areas with polysiliconprior to oxidation, a subsequent oxidation step causes less mesaconsumption and minimizes undesirable widening of the trench.

In an alternative embodiment, simplified cross-sectional views of whichare shown in FIGS. 46A, 46B and 46C, the cavity inside the trenchresulting from the shield poly recess etch is filled with a dielectricfill material 4608F having similar etch rate as the etch rate of shielddielectric 4608S. This step may be carried out using any one of highdensity plasma (HDP) oxide deposition, chemical vapor deposition (CVD)or spin-on glass (SOG) processes, followed by a planarization step toobtain a planar surface at the top of the trench. Dielectric fillmaterial 4608F and shield dielectric material 4608S are then uniformlyetched back such that a layer of insulating material having therequisite thickness remains over shield electrode 4611 as shown in FIG.46B. The trench sidewalls are then lined with gate dielectric afterwhich the remaining trench cavity is filled with gate electrode as shownin FIG. 46C. The result is a highly conformal IPD layer that is free oftopographical non-uniformities.

An exemplary embodiment for another method of forming high quality IPDis shown in the simplified cross-sectional views of FIGS. 47A and 47B.After the formation of shield dielectric layer 4708S inside the trenchand filling the cavity with shield poly 4711, a shield poly etch backstep is performed to recess the shield poly inside the trench. In thisembodiment, the shield poly recess etch leaves more poly in the trenchsuch that the top surface of the recessed shield poly is higher than thefinal target depth. The thickness of the extra poly on the top surfaceof the shield poly is designed to be approximately the same as thetarget IPD thickness. This upper portion of the shield electrode is thenphysically or chemically altered to further enhance its oxidation rate.A method to chemically or physically alter the electrode can beperformed by ion implanting impurities such as fluorine or argon ionsinto the polysilicon to enhance the oxidation rate of the shieldelectrode, respectively. The implant is preferably performed at zerodegrees, i.e., perpendicular to the shield electrode as shown in FIG.47A, so as not to physically or chemically alter the trench sidewalls.Next, shield dielectric 4708S is etched to remove the dielectric fromthe trench sidewalls. This shield dielectric recess etch causes a slightrecess in the remaining shield dielectric adjacent shield electrode 4711(similar to that shown in FIG. 45A). This is followed by a conventionaloxidation step whereby the altered top portion of shield poly 4711oxidizes at a faster rate than the sidewalls of the trench. This resultsin the formation of a substantially thicker insulator 4708T over theshield electrode than along the sidewalls of the trench silicon surface.The thicker insulator 4708T over the shield electrode forms the IPD. Thealtered poly oxidizes in the lateral direction as well compensating forsome of the trough formed in the top surface of the shield dielectric asa result of the shield dielectric recess etch. Conventional steps arethen carried out to form the gate electrode in the trench resulting inthe structure shown in FIG. 47B. In one embodiment, the shield electrodeis altered to obtain an IPD-to-gate oxide thickness ratio in the rangeof 2-to-1 to 5-to-1. As an example, if a 4-to-1 ratio is selected, forabout 2000 Å of IPD formed over the shield electrode, about 500 Å ofgate oxide is formed along the trench sidewalls.

In an alternative embodiment, the physical or chemical alternation stepis carried out after a shield dielectric recess etch. That is, shieldoxide 4708S is etched to remove the oxide from the trench sidewalls.This exposes the upper portion of the shield electrode and the siliconto a physical or chemical alteration method as described above. With thetrench sidewalls exposed, the alteration step is confined to horizontalsurfaces, i.e. silicon mesa and shield electrode only. The alterationmethod, such as ion implanting of dopants, would be performed at zerodegrees (perpendicular to the shield electrode) so as not to physicallyor chemically alter the trench sidewall. Conventional steps are thencarried out to form the gate electrode in the trench thus resulting in athicker dielectric over the shield electrode.

Yet another embodiment for forming an improved IPD layer is shown inFIG. 48. According to this embodiment, a thick insulator layer 4808Tmade of, e.g., oxide, is formed over the recessed shield oxide 4808S andshield electrode 4811. Thick insulator 4808T is preferentially formed(i.e., “bottom up fill”) using such directional deposition techniques ashigh density plasma (HDP) deposition or plasma-enhanced chemical vapordeposition (PECVD). Directional deposition results in the formation of asubstantially thicker insulator along the horizontal surfaces (i.e.,over the shield electrode and the shield oxide) than along the verticalsurfaces (i.e., along the trench sidewalls) as shown in FIG. 48. An etchstep is then performed to remove the oxide off the sidewalls, whileleaving sufficient oxide over the shield polysilicon. Conventional stepsare then carried out to form the gate electrode in the trench. Anadvantage of this embodiment, other than obtaining a conformal IPD, isthat mesa consumption and trench widening is prevented because the IPDis formed through a deposition process rather than an oxidation process.Another benefit of this technique is the rounding obtained at the topcorners of the trench.

In another embodiment, after the shield dielectric and shield polyrecess a thin layer of screen oxide 4908P is grown inside the trench.Then, a layer of silicon nitride 4903 is deposited to cover screen oxide4908P as shown in FIG. 49A. Silicon nitride layer 4903 is thenanisotropically etched such that it is removed from the bottom surfaceof the trench (i.e., above shield poly) but not from the trenchsidewalls. The resulting structure is shown in FIG. 49B. The wafer isthen exposed to an oxidizing ambient, causing a thick oxide 4908T toform on the shield polysilicon surface as shown in FIG. 49C. Sincenitride layer 4903 is resistant to oxidation, no significant oxidegrowth occurs along the trench sidewalls. Nitride layer 4903 is thenremoved by wet etching, using for example hot phosphoric acid.Conventional process steps follow to form the gate oxide and gatedielectric, as shown in FIG. 49D.

In some embodiments the formation of the IPD layer involves an etchprocess. For example, for embodiments where the IPD film is depositedover topography a film layer much thicker than the desired final IPDthickness may be deposited first. This is done to get a planar filmlayer to minimize the dishing of the starting layer into the trenches.The thicker film, which may completely fill the trench and extend overthe silicon surface, is then etched to reduce its thickness to thetarget IPD layer thickness. According to one embodiment, this IPD etchprocess is performed in at least two etch steps. The first step isintended to planarize the film back to the silicon surface. In this stepthe uniformity of the etch is important. The second step is intended torecess the IPD layer to the desired depth (and thickness) within thetrench. In this second step, the etch selectivity of the IPD film tosilicon is important. During the recess etch step the silicon mesa isexposed, as well as the silicon trench sidewall as the IPD layer isrecessed into the trench. Any loss of silicon on the mesa affects theactual trench depth and, if a T-trench is involved, the depth of the Tis also affected.

In one exemplary embodiment shown in FIG. 50A, an anisotropic plasmaetch step 5002 is used to planarize the IPD film down to the surface ofthe silicon. An exemplary etch rate for the plasma etch may be 5000A/min. This is followed by an isotropic wet etch 5004 to recess the IPDinto the trench. The wet etch is preferably performed using a controlledsolution selective to silicon so as not to attack the silicon sidewallwhen exposed and to provide a repeatable etch to obtain a specificrecess depth. An exemplary chemistry for the wet etch may be 6:1buffered oxide etch (BOE) which produces an etch rate of about 1100A/min at 25C. Commonly-assigned U.S. Pat. No. 6,465,325 to RodneyRidley, which is hereby incorporated by reference in its entirety,provides details for exemplary plasma and wet etch recipes suitable forthis process. The first plasma etch step for planarization results inless dishing of the IPD layer over the trenches than would a wet etch.The second wet etch step for the recess etch results in betterselectivity to silicon and less damage to the silicon than would occurwith a plasma etch. In an alternative embodiment shown in FIG. 50B, achemical mechanical planarization (CMP) process is used to planarize theIPD film down to the silicon surface. This is followed by a wet etch torecess the IPD into the trench. The CMP process results in less dishingof the IPD layer over the trenches. The wet etch step for the recessetch results in better selectivity to silicon and less damage to thesilicon that would occur with CMP. Other combinations of these processesare also possible.

Formation of a high quality insulating layer is desirable in structuresother than the IPD, including the trench and planar gate dielectric,inter-layer dielectric and the like. The most commonly used dielectricmaterial is silicon dioxide. There are several parameters that define ahigh quality oxide film. The primary attributes are uniform thickness,good integrity (low interface trap density), high electric fieldbreakdown strength, and low leakage levels, among others. One of thefactors that impacts many of these attributes is the rate at which theoxide is grown. It is desirable to be able to accurately control thegrowth rate of the oxide. During thermal oxidation, there is a gas phasereaction with charged particles on the wafer surface. In one embodiment,a method for controlling oxidation rate is implemented by influencingthe charge particles, typically silicon and oxygen, by the applicationof an external potential to the wafer to either increase or decrease therate of oxidation. This differs from the plasma enhanced oxidation inthat no plasma (with reactive species) is created above the wafer. Also,according to this embodiment the gas is not accelerated toward thesurface; it is merely prevented from reacting with the surface. In anexemplary embodiment, a reactive ion etch (RIE) chamber with hightemperature capability can be used to regulate the level of energyneeded. The RIE chamber is used not for etching, but for applying a DCbias to control the energy needed to slow and stop oxidation. FIG. 51 isa flow diagram for an exemplary method according to this embodiment.Initially, the RIE chamber is used to apply a DC bias to the wafer in atest environment (5100). After determining the potential energy neededto inhibit the surface reaction (5200), an external bias is applied thatis large enough to prevent oxidation from occurring (5120). Then, bymanipulating the external bias, such as pulsing or other methods, therate of oxidation at even extremely high temperatures can be controlled(5130). This method allows for obtaining the benefits of hightemperature oxidation (better oxide flow, lower stress, elimination ofdifferential growth on various crystal orientations, etc.) without thedrawback of rapid and non-uniform growth.

While techniques such as those described above in connection with FIG.51 can improve the quality of the resulting oxide layer, oxidereliability remains a concern especially in trench-gated devices. One ofthe main degradation mechanisms is due to high electric fields at thetrench corners, which results from localized thinning of the gate oxideat these points. This leads to high gate leakage currents and low gateoxide breakdown voltage. This effect is expected to become more severeas trench devices are further scaled to reduce on-resistance, and asreduced gate voltage requirements lead to thinner gate oxides.

In one embodiment, concerns with gate oxide reliability are alleviatedby using dielectric materials with higher dielectric constant (high-Kdielectrics) than silicon-dioxide. This allows equivalent thresholdvoltage and transconductance with a much thicker dielectric. Accordingto this embodiment, the high-K dielectric reduces gate leakage andincreases the gate dielectric breakdown voltage, without degradation ofthe device on-resistance or drain breakdown voltage. High-K materialsthat exhibit the required thermal stability and suitable interface-statedensities to be integrated into trench-gated and other power devicesinclude Al₂O₃, HfO₂, Al_(x)HfyO_(z), TiO₂, ZrO₂ and the like.

As discussed above, to improve the switching speed of a trench gatedpower MOSFET it is desirable to minimize the transistor gate-to-draincapacitance Cgd. Using a thicker dielectric layer at the bottom of thetrench as compared to the trench sidewalls is one of several methodsdescribed above for reducing Cgd. One method for forming a thick bottomoxide layer involves the formation of a thin layer of screen oxide alongthe sidewalls and the bottom of the trench. The thin oxide layer is thencovered by a layer of oxidization-inhibiting material such as nitride.The nitride layer is then etched anisotropically, such that all thenitride is removed from the horizontal bottom surface of the trench butthe trench sidewalls remain coated by the nitride layer. After theremoval of the nitride from the bottom of the trench, an oxide layerhaving the desirable thickness is formed at the bottom of the trench.Thereafter, a thinner channel oxide layer is formed after the removal ofthe nitride and screen oxide from the trench sidewalls. This method forforming a thick bottom oxide and variations thereof are described ingreater detail in commonly-assigned U.S. Pat. No. 6,437,386, to Hurst etal., which is hereby incorporated in its entirety. Other methods offorming a thick oxide at the bottom of a trench involving selectiveoxide deposition are described in commonly-owned U.S. Pat. No. 6,444,528to Murphy, which is hereby incorporated in its entirety.

In one embodiment, an improved method of forming thick oxide at thebottom of a trench uses a sub-atmospheric chemical vapor deposition(SACVD) process. According to this method, an exemplary flow diagram forwhich is shown in FIG. 52, after etching the trench (5210), SACVD isused to deposit a highly conformal oxide film (5220), using for examplethermal Tetraethoxyorthsilane (TEOS) that fills the trench without voidsin the oxide. The SACVD step can be carried out at sub-atmosphericpressures ranging from 100 Torr to 700 Torr, and at an exemplarytemperature in the range from about 450° C. to about 600° C. The TEOS(in mg/min) to Ozone (in cm³/min) ratio can be set between the range of,for example, 2 to 3, preferably about 2.4. Using this process, an oxidefilm having a thickness anywhere from about 2000 Å to 10,000 Å orgreater can be formed. It is to be understood that these numbers are forillustrative purposes only and may vary depending on the specificprocess requirements and other factors such as the atmospheric pressureof the location of the fabrication facility. The optimal temperature maybe obtained by balancing the rate of deposition with the quality of theresulting oxide layer. At higher temperatures the deposition rate slowsdown which may reduce film shrinkage. Such film shrinkage can cause agap to form in the oxide film in the center of the trench along theseam.

After the oxide film is deposited, it is etched back from the siliconsurface and inside the trench to leave a relatively flat layer of oxidewith the desired thickness at the bottom of the trench (5240). This etchcan be preformed by a wet etch process, or a combination of wet and dryetch processes, using for example, diluted HF. Because the SACVD-formedoxide tends to be porous it absorbs ambient moisture after deposition.In a preferred embodiment, a densification step 5250 is performedfollowing the etch-back step to ameliorate this effect. Densificationcan be performed by temperature treatment at, for example, 1000° C. forabout 20 minutes.

An added benefit to this method is the ability to mask off (step 5230)an end trench during the etch-back step of the SACVD oxide, leaving anoxide-filled termination trench. That is, for the various embodiments oftermination structures described above that include a dielectric-filledtrench, the same SACVD step can be used to fill the termination trenchwith oxide. Also, by masking the field termination region duringetch-back, the same SACVD process step can result in the formation offield oxide in the termination region, eliminating otherwise requiredprocess steps to form thermal field oxide. Furthermore, this processprovides additional flexibility as it allows a complete reworking ofboth the termination dielectric layer and the thick bottom oxide in caseit is etched too far since silicon is not consumed by thermal oxidationprocess but instead provided in both locations during the SACVDdeposition.

In another embodiment, another method for forming thick oxide at thebottom of the trench uses a directional TEOS process. According to thisembodiment, an exemplary flow diagram for which is shown in FIG. 53, theconformal properties of TEOS are combined with the directional nature ofplasma enhanced chemical vapor deposition (PECVD) to selectively depositoxide (5310). This combination enables a higher deposition rate onhorizontal surfaces than vertical surfaces. For example, an oxide filmdeposited using this process may have a thickness of about 2500 Å at thebottom of the trench and an average thickness of about 800 Å on thetrench sidewalls. The oxide is then isotropically etched until all theoxide from the sidewalls is removed, leaving a layer of oxide at thebottom of the trench. The etch process may include a dry top oxide etchstep 5320 followed by a wet buffered oxide etch (BOE) step 5340. For theexemplary embodiment described herein, after the etch there remains alayer of oxide at the bottom of the trench having a thickness of, e.g.,1250 Å with all sidewall oxide removed.

In a specific embodiment, a dry top oxide etch is employed thatconcentrates on the top surface of the structure, etching the oxide offthe top area at an accelerated rate while etching the oxide in thebottom of the trench at a much reduced rate. This type of etch, referredto herein as “fog etch” involves a careful balancing of the etchconditions and etch chemistry to yield the desired selectivity. In oneexample, this etch is performed at a relatively low power and lowpressure using a plasma etcher with a top power source such as the LAM4400. Exemplary values for the power and pressure may be anywhere in therange of 200-500 Watts and 250-500 mTorr, respectively. Different etchchemistries can be used. In one embodiment, a combination of a fluorinechemistry, e.g., C2F6, and chlorine, mixed at an optimal ratio of, forexample, about 5:1 (e.g., C2F6 at 190 sccm and Cl at 40 sccm), yieldsthe desired selectivity. Using chlorine as part of oxide etch chemistryis unusual because chlorine is more commonly used for etching metal orpolysilicon and it normally inhibits etching of oxide. However, forpurposes of this type of selective etch, this combination works wellbecause the C2F6 aggressively etches the oxide near the top surfacewhere the higher energy allows the C2F6 to overcome the impact of thechlorine, while closer to the trench bottom chlorine slows down the etchrate. This primary dry etch step 5320 maybe followed by a cleanup etch5330 prior to the BOE dip 5340. It is to be understood that according tothis embodiment, the optimal selectivity is achieved by fine tuning thepressure, energy, and etch chemistry which may vary depending on theplasma etch machine.

The PECVD/etch process according to this embodiment, can be repeated oneor more times if desired to obtain a bottom oxide with the targetthickness. This process also results in the formation of thick oxide onthe horizontal mesa surface between trenches. This oxide can be etchedafter polysilicon is deposited in the trenches and etched back on thesurface, so that the trench bottom oxide is protected from thesubsequent etch step.

Other methods for selectively forming thick oxide at the bottom of thetrench are possible. FIG. 54 shows a flow diagram for one exemplarymethod that uses high density plasma (HDP) deposition to keep oxide frombuilding up on the trench sidewalls (5410). A property of the HDPdeposition is that it etches as it deposits, resulting in less of anoxide buildup on the trench sidewalls relative to the oxide on thetrench bottom, as compared to the directional TEOS method. A wet etch(step 5420) can then be employed to remove some or clear the oxide fromthe sidewalls, while leaving a thick oxide on the trench bottom. Anadvantage of this process is that the profile at the top of the trenchslopes away (5510) from the trench (5500) as shown in FIG. 55, makingvoid-free poly fill easier to achieve. A “fog etch” (step 5430) asdescribed above can be employed to etch some oxide off the top beforepoly fill (step 5440) so that less oxide would need to be etched fromthe top after poly etch. The HDP deposition process can also be used todeposit oxide between two poly layers in a trench with buried electrodes(e.g., trench MOSFETs with shielded gate structures).

According to yet another method shown in FIG. 56, a selective SACVDprocess is used to form a thick oxide on the trench bottom. This methodmakes use of the ability of SACVD to become selective at a lowerTEOS:Ozone ratio. Oxide has an extremely slow deposition rate on siliconnitride but deposits readily on silicon. The lower the ratio of TEOS toOzone, the more selective the deposition becomes. According to thismethod, after etching the trenches (5610), pad oxide is grown on thesilicon surface of the trench array (5620). A thin layer of nitride isthen deposited on the pad oxide (5630). This is followed by ananisotropic etch to remove the nitride from horizontal surfaces leavingnitride on the trench sidewalls (5640). Selective SACVD oxide is thendeposited (5650) on horizontal surfaces including the trench bottom at aTEOS:Ozone ratio of, for example, about 0.6, at about 405° C. The SACVDoxide is then optionally densified by temperature treatment (5660). Anoxide-nitride-oxide (ONO) etch is then performed to clear nitride andoxide on the sidewalls of the trench (5670).

As discussed previously, one reason for the use of a thicker oxide layerat the bottom of the gate trench as compared to its sidewalls is toreduce Qgd or gate-to-drain charge which improves switching speed. Thesame reason dictates that the depth of the trench be about the same asthe depth of the well junction to minimize trench overlap into the driftregion. In one embodiment, a method for forming a thicker dielectriclayer at the bottom of a trench extends the thicker dielectric layer upthe sides of the trench. This makes the thickness of the bottom oxideindependent of the trench depth and the well junction depth, and allowsthe trench and the poly inside the trench to be deeper than the welljunction without appreciably increasing Qgd.

An exemplary embodiment for a method of forming a thick bottomdielectric layer according to this method is shown in FIGS. 57 to 59.FIG. 57A illustrates a simplified and partial cross-section of a trenchlined with a thin layer of pad oxide 5710 and nitride layer 5720 afterit has been etched to cover only the sidewalls of the trench. Thisenables the etching of pad oxide 5710 to expose the silicon at thebottom of the trench and top surface of the die as shown in FIG. 57B.This is followed by an anisotropic etch of the exposed silicon resultingin a structure as shown in FIG. 58A, wherein both top silicon and thesilicon at the bottom of the trench have been removed to the desireddepth. In an alternative embodiment, the silicon on the top silicon canbe masked such that during silicon etch, only the bottom of the trenchis etched. Next, an oxidation step is performed to grow thick oxide 5730in locations not covered by nitride layer 5720 resulting in thestructure shown in FIG. 58B. The oxide thickness may be, for example,about 1200 Å to 2000 Å. Nitride layer 5720 is then removed and pad oxide5710 is etched away. The etching of the pad oxide will cause somethinning of thick oxide 5730. The rest of the process can employ thestandard flow to form the gate poly and well and source junctionsresulting in the exemplary structure shown in FIG. 59.

As shown in FIG. 59, the resulting gate oxide includes a bottom thicklayer 5730 that extends along the sidewalls of the trench to above thewell junction in region 5740. In some embodiments, wherein the channeldoping in the well region alongside the trench is graded with lighterdoping near the drain side 5740, this region would normally have a lowerthreshold voltage compared to the region near the source. Extending thethicker oxide along the sides of the trench overlapping into the channelin region 5740 would therefore not increase the device thresholdvoltage. That is, this embodiment allows optimizing the well junctiondepth and sidewall oxide to minimize Qgd without adversely impacting thedevice on-resistance. Those skilled in this art will appreciate thatthis method of forming thick oxide at the bottom of trench can beapplied to the variety of the devices described above including theshielded gate, dual gate in combination with the various chargebalancing structures, as well as any other trench gate devices.

Those skilled in the art will also appreciate that any of the aboveprocesses for forming a thick oxide at the bottom of a trench and forIPD can be employed in the process for forming any of the trench gatedtransistors described herein. Other variations for these processes arepossible. For example, as in the case of the process described inconnection with FIGS. 47A and 47B, chemical or physical alteration ofthe silicon can enhance its oxidation rate. According to one suchexemplary embodiment, a halogen ion species, e.g., fluorine, bromine,etc., is implanted at a zero angle into the silicon at the bottom of thetrench. The implant may occur at an exemplary energy of about 15 KeV orless, at an exemplary dose greater than 1E¹⁴ (e.g., 1E¹⁵ to 5E¹⁷), andat an exemplary temperature between the range of 900° C. to 1150° C. Inthe halogen implanted areas at the trench bottom oxide grows at anaccelerated rate as compared to the trench sidewalls.

A number of the trench devices described above include trench sidewalldoping for charge balance purposes. For example, all of the embodimentsshown in FIGS. 5B and 5C, and 6 through 9A have some type of trenchsidewall doping structure. Sidewall doping techniques are somewhatlimited due to the physical constraints of narrow, deep trenches and/orperpendicular sidewall of the trench. Gaseous sources or angled implantscan be used to form the trench sidewall doped regions. In oneembodiment, an improved trench sidewall doping technique uses plasmadoping or pulsed-plasma doping technology. This technology utilizes apulsed voltage that is applied to a wafer encompassed in a plasma ofdopant ions. The applied voltage accelerates the ions from a cathodesheath toward and into the wafer. The applied voltage is pulsed and theduration continued until the desired dose is achieved. This techniqueenables implementing many of these trench devices with conformal dopingtechniques. Additionally, the high throughput of this process reducesthe overall cost of the manufacturing process.

Those skilled in the art will appreciate that the use of plasma dopingor pulsed-plasma doping technology is not limited to trench chargebalance structures, but can also be applied to other structures,including trenched termination structures and trenched drain, source orbody connections. For example, this methodology can be used to dope thetrench sidewalls of shielded trench structures such as those describedin connection with FIGS. 4D, 4E, 5B, 5C, 6, 7, 8, and 9A. In addition,this technique can be used to create a uniformly-doped channel region.The penetration of the depletion region into the channel region (p-welljunction) when the power device is reverse biased is controlled by thecharge concentration on both sides of the junction. When the dopingconcentration in the epi layer is high, depletion into the junction canallow punch-through to limit the breakdown voltage or require a longerchannel length than desired to maintain low on-resistance. To minimizethe depletion into the channel, higher channel doping concentration maybe required which can cause the threshold to increase. Since thethreshold is determined by the peak concentration below the source in atrench MOSFET, a uniform doping concentration in the channel can providea better trade-off between channel length and breakdown.

Other methods that can be employed to obtain more uniform channelconcentration include forming the channel junction using an epitaxialprocess, using multiple energy implants, and other techniques forcreating an abrupt junction. Another technique employs a starting waferwith a lightly doped cap layer. In this way compensation is minimizedand up diffusion can be harnessed to create a more uniform channeldoping profile.

A trench device can take advantage of the fact that the threshold is setby the channel doping concentration along the trench sidewalls. Aprocess which allows a high doping concentration away from the trencheswhile maintaining a low threshold can help to prevent the punch-throughmechanism. Providing the p-well doping before the gate oxidation processallows for segregation of well p-type impurities, e.g., boron, into thetrench oxide to reduce the concentration in the channel, thus reducingthe threshold. Combining this with the techniques above can provide ashorter channel length without punch-through.

Some power applications require measuring the amount of current flowingthrough the power transistor. This is typically accomplished byisolating and measuring a portion of the total device current that isthen used to extrapolate the total current flowing through the device.The isolated portion of the total device current flows through a currentsensing or detecting device that generates a signal which is indicativeof the magnitude of the isolated current and which is then used todetermine the total device current. This arrangement is commonly knownas a current mirror. The current sensing transistor is usuallyfabricated monolithically with the power device with both devicessharing a common substrate (drain) and gate. FIG. 60 is a simplifieddiagram of a MOSFET 6000 with a current sense device 6002. The currentflowing through the main MOSFET 6000 is divided between the maintransistor and current sense portion 6002 in proportion to the activeareas of each. The current flowing through the main MOSFET is thuscalculated by measuring the current through the sense device and thenmultiplying it by the ratio of the active areas.

Various methods for isolating the current sense device from the maindevice are described in commonly-owned U.S. patent application Ser. No.10/315,719, entitled “Method of Isolating the Current Sense on PowerDevices While Maintaining a Continuous Strip Cell,” to Yedinak et al.,which is hereby incorporated in its entirety. Embodiments forintegrating the sense device along with various power devices, includingthose with charge balancing structures, are described hereinafter.According to one embodiment, in a power transistor with charge balancestructures and a monolithically integrated current sense device, thecurrent sense area is preferably formed with the same continuous MOSFETstructure as well as the charge balance structure. Without maintainingcontinuity in the charge balance structure, the device breakdown voltagewill be degraded due to a mismatch in charge causing the voltagesupporting region to be not fully depleted. FIG. 61A shows one exemplaryembodiment for a charge balance MOSFET 6100 with a planar gate structureand isolated current sense structure 6115. In this embodiment, thecharge balance structure includes opposite conductivity (in this examplep-type) pillars 6126 formed inside (n-type) drift region 6104. P-typepillars 6126 can be formed, for example, as doped polysilicon or epifilled trenches. As depicted in FIG. 61A, the charge balance structuresmaintain continuity under current sense structure 6115. Sense pad metal6113 covering the surface area of current sense device 6115 iselectrically separated from source metal 6116 by dielectric region 6117.It is to be understood that current sense devices with similarstructures can be integrated with any of the other power devicesdescribed herein. For example, FIG. 61B shows an example of how acurrent sense device can be integrated with a trench MOSFET withshielded gate where charge balancing can be obtained by adjusting thedepth of the trench and biasing the shield poly inside the trench.

There are a number of power applications where it is desirable tointegrate diodes on the same die as the power transistor. Suchapplications include temperature sensing, electro-static discharge (ESD)protection, active clamping, and voltage dividing among others. Fortemperature sensing, for example, one or more series connected diodesare monolithically integrated with the power transistor whereby thediode's anode and cathode terminals are brought out to separate bondpads, or connected to monolithic control circuit components usingconductive interconnections. The temperature is sensed by the change inthe diode (or diodes) forward voltage (Vf). For example, withappropriate interconnection to the gate terminal of the powertransistor, as the diode Vf drops with temperature, the gate voltage ispulled low reducing the current flowing through the device until thedesired temperature is achieved.

FIG. 62A shows an exemplary embodiment for a MOSFET 6200A with seriestemperature sensing diodes. MOSFET 6200A includes a diode structure 6215wherein doped polysilicon with alternating conductivity form threeseries temperature sensing diodes. In this illustrative embodiment, theMOSFET portion of device 6200A employs p-type epi-filled charge balancetrenches forming opposite conductivity regions inside n-type epi driftregion 6204. As depicted, the charge balance structure preferablymaintains continuity under temperature sense diode structure 6215. Thediode structure is formed on top of a field dielectric (oxide) layer6219 atop the surface of the silicon. A p-type junction isolation region6221 can be optionally diffused under dielectric layer 6219. A device6200B without this p-type junction is shown in FIG. 62B. To make surethat series forward biased diodes are obtained, shorting metal 6223 isused to short the P/N+ junctions that are reversed biased. In oneembodiment, p+ is implanted and diffused across the junctions to form aN+/P/P+/N+ structure where p+ appears under shorting metals 6223 toobtain improved ohmic contact. For the opposite polarity N+ can also bediffused across the N/P+ junction to form P+/N/N+/P+ structure. Again,those skilled in the art will appreciate that this type of temperaturesensing diode structure can be employed in any one of the various powerdevices in combination with many of the other features described herein.FIG. 62C, for example, depicts a MOSFET 6200C with a shielded trenchgate structure, where the shield poly can be used for charge balancing.

In another embodiment, by employing similar isolation techniques asshown in devices 6200 for temperature sensing diodes, asymmetrical ESDprotection is implemented. For ESD protection purposes, one end of thediode structure is electrically connected to the source terminal and theother end to the gate terminal of the device. Alternatively, symmetricalESD protection is obtained by not shorting any of the back to backN+/P/N+ junctions as shown in FIGS. 63A and 63B. The exemplary MOSFET6300A shown in FIG. 63A employs a planar gate structure and usesopposite conductivity pillars for charge balancing, while exemplaryMOSFET 6300B shown in FIG. 63B is a trench gate device with a shieldedgate structure. To prevent non-uniformities in charge balance, thecharge balance structure is continued under gate bond pad metal and anyother control element bond pads.

Exemplary ESD protection circuits are shown in FIGS. 64A to 64D whereinthe main device, the gate of which is being protected by the diodestructures described above, can be any one of the power devicesdescribed herein using any one of the charge balancing or othertechniques. FIG. 64A shows a simplified diagram for an asymmetricalisolated poly diode ESD protection, while FIG. 64B depicts a standardback to back isolated poly diode ESD protection circuit. The ESDprotection circuit shown in FIG. 64C uses an NPN transistor for BV_(cer)snap-back. The subscript “cer” in BV_(cer) refers to a reverse biasedcollector-emitter bipolar transistor junction in which a connection tothe base uses a resistor to control the base current. A low resistancecauses most of the emitter current to be removed through the basepreventing the emitter-base junction from turning-on, that is, injectingminority carriers back into the collector. The turn-on condition can beset by the resistor value. When carriers are injected back into thecollector the sustaining voltage between the emitter and collector isreduced—a phenomenon referred to as “snap-back.” The current at whichthe BV_(cer) snap-back is triggered can be set by adjusting the value ofthe base-emitter resistor R_(BE). FIG. 64D shows an ESD protectioncircuit that uses a silicon-controlled rectifier or SCR and diode asshown. By using a gate cathode short structure, the trigger current canbe controlled. The diode breakdown voltage can be used to offset thevoltage at which the SCR latches. The monolithic diode structure asdescribed above can be employed in any of these and other ESD protectioncircuits.

In some power applications, an important performance characteristic of apower switching device is its equivalent series resistance or ESR thatis a measure of the impedance of the switching terminal or gate. Forexample, in synchronous buck converters using power MOSFETs, lower ESRhelps reduce switching losses. In the case of trench gated MOSFETs,their gate ESR is determined in large part by the dimenstions of thepolysilicon filled trenches. For example, the length of the gatetrenches may be constrained by package limitations such as the minimumwire bond pad size. It is known that applying a silicide film topolysilicon lowers the resistance of the gate. Implementing silicidedpoly in trench MOSFETs, however, poses a number of challenges. Intypical planar discrete MOS structures, the gate poly can be silicidedafter the junctions have been implanted and driven to their respectivedepths. For trench gate devices where the gate poly is recessed,applying silicide becomes more complicated. The use of conventionalsilicide limits the maximum temperature a wafer can be subjected toopost silicide treatment to approximately less than 900° C. This places asignificant constraint on the stage of the fabrication process whendiffused regions such as sources, drains and wells, are formed. The mosttypical metal used for silicides is titanium. Other metals such astungsten, tantalum, cobalt and platinum can also be used allowing ahigher thermal budget post silicide processing which provides moreprocessing latitude. The gate ESR can also be reduced by various layouttechniques.

Described below are various embodiments for forming charge balancedpower switching devices with lower ESR. In one embodiment shown in FIG.65, a process 6500 includes forming trenches with a lower electrodeformed at a lower portion of the trench for shielding and/or chargebalance purposes (step 6502). This is followed by depositing and etchingan IPD layer (step 6504). The IPD layer can be formed by knownprocesses. Alternatively, any one of the processes described above inconnection with FIGS. 45 to 50 can be used to form the IPD layer. Next,an upper electrode or gate poly is deposited and etched at step 6506using known processes. This is followed by implanting and driving thewell and source regions (step 6508). It is after step 6508 that silicideis applied to the gate poly at step 6510. This is then followed bydeposition and planarization of a dielectric at step 6512. In avariation of this process, step 6512, where the dielectric field isdeposited and planarized, is preformed first and then contact holes areopened to reach the source/body and the gate, after which silicidecontacts are formed. These two embodiments rely on the heavy bodyimplant region being activated by a low temperature anneal that is lowerthan the silicide film transition point.

In another embodiment, the poly gate is replaced by a metal gate.According to this embodiment, a metal gate is formed by depositing,e.g., Ti, using a collimated source to improve fill capability in atrench structure. After applying the metal gate and once the junctionshave been implanted and driven, dielectric options include HDP and TEOSto isolate the gate from source/body contacts. In alternativeembodiments, a damascene or dual damascene approach with various metaloptions from aluminum to copper top-metals is used to form the gateterminal.

The layout of the gate conductor can also affect the gate ESR andoverall switching speed of the device. In another embodiment shown inFIGS. 66A and 66B, a layout technique combines perpendicular silicidedsurface poly stripes with recessed trench poly to reduce gate ESR.Referring to FIG. 66A, a highly simplified device structure 6600 isshown wherein a silicide-coated poly line 6604 extends along the surfaceof the silicon perpendicular to trench stripes 6602. FIG. 66Billustrates a simplified cross-sectional view of device 6600 along theAA′ axis. Silicided poly line 6604 contacts the gate poly atintersections with trenches. Multiple silicided poly lines 6604 canextend atop the silicon surface to reduce the resistivity of the gateelectrode. This and other layout techniques made possible by, forexample, processes having two or more layers of interconnect, can beemployed to improve gate ESR in any one of the trench gate devicesdescribed herein.

Circuit Applications

With the dramatic reduction in the device on-resistance as provided by,for example, the various device and process techniques described herein,the chip area occupied by the power device can be reduced. As a result,the monolithic integration of these high-voltage devices with lowvoltage logic and control circuitry becomes more viable. In typicalcircuit applications, the types of functions that can be integrated onthe same die as the power transistor include power control, sensing,protection and interface circuitry. An important consideration in themonolithic integration of power devices with other circuitry is thetechnique used to electrically isolate the high voltage power devicesfrom the low voltage logic or control circuitry. There exist a number ofknown approaches to achieve this, including junction isolation,dielectric isolation, silicon-on-insulator, and the like.

Below, a number of circuit applications for power switching will bedescribed wherein the various circuit components can be integrated onthe same chip to varying degrees. FIG. 67 depicts a synchronous buckconverter (DC-DC converter) requiring lower voltage devices. In thiscircuit, n-channel MOSFET Q1, commonly referred to as the “high sideswitch,” is designed to have a moderately low on-resistance but fastswitching speed to minimize the power losses. MOSFET Q2, commonlyreferred to as the low side switch, is designed to have a very lowon-resistance and moderately high switching speed. FIG. 68 depictsanother DC-DC converter that is more suitable for medium to high voltagedevices. In this circuit, the main switching device Qa exhibits fastswitching speed, and high blocking voltage. Because this circuit uses atransformer, lower current flows through transistor Qa which allows itto have a moderately low on-resistance. For the synchronous rectifierQs, a MOSFET with low to very low on-resistance, fast switching speed,very low reverse recovery charge, and low inter-electrode capacitancecan be used. Other embodiments and improvements to such DC-DC convertersare described in greater detail in commonly-assigned U.S. patentapplication Ser. No. 10/222,481, entitled “Method and Circuit forReducing Losses in DC-DC Converters,” by Elbanhawy, which is herebyincorporated in its entirety.

Any one of the various power device structures described above can beused to implement the MOSFETs in the converter circuits of FIGS. 67 and68. The dual gate MOSFET of the type shown in FIG. 4A, for example, isone type of device that offers particular advantages when used inimplementing synchronous buck converters. In one embodiment, a specialdrive scheme takes advantage of all the features offered by the dualgate MOSFET. An example of this embodiment is shown in FIG. 69, whereina first gate terminal G2 of high side MOSFET Q1 has its potentialdetermined by the circuit made up of diode D1, resistors R1 and R2, andcapacitor C1. The fixed potential at gate electrode G2 of Q1 can beadjusted for best Qgd to optimize the switching time of the transistor.The second gate terminal G1 of high side switch transistor Q1 receivesthe normal gate drive signal from the pulse width modulated (PWM)controller/driver (not shown). The two gate electrodes of the low sideswitch transistor Q2 are similarly driven, as shown.

In an alternative embodiment, an example of which is shown in FIG. 70A,both gate electrodes of the high side switch are driven separately tofurther optimize the performance of the circuit. According to thisembodiment, different waveforms drive gate terminals G1 and G2 of highside switch Q1 to achieve best switching speed during transitions andbest on-resistance R_(DSon) during the rest of the cycle. In the exampleshown, a voltage Va of about 5 volts during switching delivers very lowQgd to the gate of high side switch Q1 resulting in high switchingspeed, but R_(DSon) before and after transitions td1 and td2 is not atits lowest value. This, however, does not adversely impact the operationof the circuit since during switching R_(DSon) is not a significant losscontributor. To ensure the lowest R_(DSon) during the rest of the pulseduration, the potential V_(g2) at gate terminal G2 is driven to a secondvoltage Vb higher than Va during time period t_(p) as shown in thetiming diagram of FIG. 70B. This driving scheme results in optimalefficiency. Variations on these driving schemes are described in greaterdetail in commonly-assigned U.S. patent application Ser. No. 10/686,859,entitled “Driver for Dual Gate MOSFETs,” by Elbanhawy, which is herebyincorporated by reference in its entirety.

Packaging Technologies

An important consideration for all power semiconductor devices is thehousing or package that is used to connect the device to the circuit.The semiconductor die is typically attached to a metal pad using eithermetal bonding layers such as solder or metal filled epoxy adhesives.Wires are usually bonded to the top surface of the chip and then toleads that protrude through the molded body. The assembly is thenmounted to a circuit board. The housing provides both electrical andthermal connections between the semiconductor chip and the electronicsystem and its environment. Low parasitic resistance, capacitance, andinductance are desired electrical features for the housing that enable abetter interface to the chip.

Improvements to the packaging technology have been proposed that focuson reducing resistance and inductance in the package. In certain packagetechnologies, solder balls or copper studs are distributed on therelatively thin (e.g., 2-5 μm) metal surface of the chip. Bydistributing the metal connections on the large area metal surface, thecurrent path in the metal is made shorter and metal resistance isreduced. If the bumped side of the chip is connected to a copper leadframe or to the copper traces on a printed circuit board the resistanceof the power device is reduced compared to a wire bonded solution.

FIGS. 71 and 72 illustrate simplified cross-sectional views of moldedand unmolded packages, respectively, using solder balls or copper studsthat connect lead frames to the metal surface of the chip. Moldedpackage 7100 as shown in FIG. 71 includes a leadframe 7106 that connectsto a first side of a die 7102 via solder balls or copper studs 7104. Thesecond side of die 7102 which faces away from leadframe 7106 is exposedthrough a molding material 7108. In typical vertical power transistors,the second side of the die forms the drain terminal. The second side ofthe die can form a direct electrical connection to a pad on the circuitboard, thus providing a low resistance thermal and electrical path forthe die. This type of package and variations thereof are described ingreater detail in commonly-assigned U.S. patent application Ser. No.10/607,633, entitled “Flip Chip in Leaded Molded Package and Method ofManufacture Thereof,” by Joshi et al., which is hereby incorporated inits entirety.

FIG. 72 shows an unmolded embodiment of a package 7200. In the exemplaryembodiment shown in FIG. 72, package 7200 has a multi-layer substrate7212 that includes a base layer 7220 comprising, e.g. metal, and a metallayer 7221 separated by an insulating layer 7222. Solder structures 7213(e.g., solder balls) are attached to substrate 7212. A die 7211 isattached to substrate 7212, with solder structures 7213 disposed aroundthe die. Die 7211 can be coupled to substrate 7212 with a die attachmaterial such as solder 7230. After the illustrated package is formed,it is flipped over and mounted onto a circuit board (not shown) or othercircuit substrate. In embodiments where a vertical power transistor isfabricated on die 7211, solder balls 7230 form the drain terminalconnection and the chip surface forms the source terminal. The reverseconnection is also possible by reversing the connection of die 7211 tosubstrate 7212. As shown, package 7200 is thin and unmolded as a moldingmaterial is not needed. Various embodiments for unmolded packages ofthis type are described in greater detail in commonly-assigned U.S.patent application Ser. No. 10/235,249, entitled “Unmolded Package for aSemiconductor Device,” by Joshi, which is hereby incorporated in itsentirety.

Alternative methods in which the top surface of the chip is connecteddirectly to the copper by either solder or conductive epoxy have beenproposed. Because the stress induced between the copper and silicon chipincreases with the area of the chip, the direct connection method may belimited since the solder or epoxy interface can only be stressed so muchbefore breaking. Bumps, on the other hand, allow for more displacementbefore breaking and have been demonstrated to work with very largechips.

Another important consideration in package design is heat dissipation.Improvements in the power semiconductor performance often result in asmaller chip area. If the power dissipation in the chip does notdecrease, the heat energy concentrates in a smaller area that can resultin a higher temperature and degraded reliability. Means to increase theheat transfer rate out of the package include reducing the number ofthermal interfaces, using materials with higher thermal conductivity,and reducing the thickness of the layers such as silicon, solder, dieattach, and die attach pad. Commonly-assigned U.S. Pat. No. 6,566,749,to Rajeev Joshi, entitled “Semiconductor Die Package with ImprovedThermal and Electrical Performance,” which is hereby incorporated in itsentirety, discusses solutions to the problems of heat dissipation,especially for dies including vertical power MOSFETs for RFapplications. Other techniques for improving overall package performanceare described in greater detail in commonly-assigned U.S. Pat. Nos.6,133,634, and 6,469,384, both to Rajeev Joshi, as well as U.S. patentapplication Ser. No. 10/271,654, entitled “Thin Thermally Enhanced FlipChip in a Leaded Molded Package,” to Joshi et al. It is to be understoodthat any one of the various power devices described herein can be housedin any of the packages described herein or any other suitable package.

Using more surfaces of the housing for heat removal also increases theability of the housing to maintain a lower temperature such as thermalinterfaces on both the top and bottom of the housing. Increased surfacearea combined with airflow around those surfaces increases the heatremoval rate. The housing design could also enable easy interface withan external heat sink. While thermal conduction and infrared radiationtechniques are the common methods, application of alternate coolingmethods are possible. For example, thermionic emission as described incommonly-assigned U.S. patent application Ser. No. 10/408,471, entitled“Power Circuitry With A Thermionic Cooling System,” by Reno Rossetti,which is hereby incorporated by reference, is one method of heat removalthat can be used to cool down power devices.

Integration of other logic circuitry including power delivery andcontrol functions in a single package presents additional challenges.For one, the housing requires more pins to interface with otherelectronic functions. The package should allow for both high currentpower interconnects in the package and low current signalinterconnections. Various packaging technologies that can address thesechallenges include chip-to-chip wire bonding to eliminate specialinterface pads, chip-on-chip to save space inside the housing, andmulti-chip modules that allow distinctive silicon technologies to beincorporated into a single electronic function. Various embodiments formulti-chip package techniques are described in commonly-assigned U.S.patent application Ser. No. 09/730,932, entitled “Stacked Package UsingFlip Chip in Leaded Molded Package Technology,” by Rajeev Joshi, and No.10/330,741, entitled “Multichip Module Including Substrate with an Arrayof Interconnect Structures,” also by Rajeev Joshi, both of which arehereby incorporated by reference in their entirety.

While the above provides a complete description of the preferredembodiments of the invention, many alternatives, modifications, andequivalents are possible. For example, many of the charge balancingtechniques are described herein in the context of a MOSFET and inparticular a trench gated MOSFET. Those skilled in the art willappreciate that the same techniques can apply to other types of devices,including IGBTs, thyristors, diodes and planar MOSFETs, as well aslateral devices. For this and other reasons, therefore, the abovedescription should not be taken as limiting the scope of the invention,which is defined by the appended claims.

1. A semiconductor device comprising: a drift region of a firstconductivity type; a well region extending above the drift region andhaving a second conductivity type opposite the first conductivity type;a plurality of active trenches extending through the well region andinto the drift region, the plurality of active trenches defining anactive area, wherein inside each of the plurality of active trenches isformed: a first conductive gate electrode disposed along and insulatedfrom a first trench sidewall, a second conductive gate electrodedisposed along and insulated from a second trench sidewall, and aconductive shield electrode disposed between the first and secondconductive gate electrodes, wherein the shield electrode is insulatedfrom and extends deeper inside the trench than the first and secondconductive gate electrodes; and source regions having the firstconductivity type formed inside the well region and adjacent theplurality of active trenches, wherein, electrical contact to theconductive shield electrode is made inside the active area, whereincontact to the well region is made via a body trench that is etched intoa central portion of a mesa between adjacent active trenches.
 2. Thesemiconductor device of claim 1 wherein the body trench extends deeperinside the well region than the source regions.
 3. The semiconductordevice of claim 1 wherein the body trench extends deeper than the wellregion.
 4. The semiconductor device of claim 1 further comprising aregion of high dopant concentration having the second conductivity typelocated at least under the body trench.